Add library product (zip with webodf.js + API dox), normalize product build targets, "zip" build target flagged for removal

I propose to have on CI runs all the build products made available on the resulting page. Not only the products targetted at 3rd-party, but also benchmarks and browser tests which are usually only interesting to WebODF developers and possibly not part of release stuff. (Or should we also include benchmarks and tests in the created tarballs/zips? No idea, at least I have not yet seen unit test packages, but maybe I just did not search for it)

Currently the CI result page shows/links
* the localeditor
* the benchmarks.

The XPI is also available, but only by url-hacking right now. It seems the plan is to add it soon. The viewer is not yet available, because the used target in DEPENDS was wrong.

This PR adds the viewer plugin (but without a proper test document, any proposals?) to the created ZIP file. It does not yet add the tests (as those are not yet copied into the build dir to be used with the compiled webodf.js and I screwed up writing cmake code for that and it got late and I first want feedback now in general).

Additionally it adds a special CIZip target, which should be used in the future to create a ZIP file with all the files/products wanted for the CI, which might be more/different from the ZIP files intended for 3rd-party.

So please feedback on these questions:

Q1: What test document(s?) should be used for the viewer?

Q2: Should there be a separate CI ZIP product, or should benchmarks and tests also be put into the to-be-released single ZIP file? I would think No, but your experience might be bigger.

**Update:**

We got further in the discussion. Idea is now to have special product targets in the build system which result in creating the official products. On the CI these official products would be build and published on the result page, so everyone could download and try (where it makes sense, JS files, API dox, XPI and APK would work, CI-platform dependent like possible binaries might not).
Additionally internal products would be built on the CI as well (benchmarks, test webpages) and published on the result page as well.

So with this PR there is now a new make target "webodf.js.zip-product" which creates a ZIP file with
* webodf.js
* webodf-debug.js
* API dox
* License
(* README yet to be done)

And there is a new make target "CI" which should be used only on the CI. This target will create the two current releasable products (webodf.js zip & Firefox extension), to catch any regressions in creating the products. And it will create a ZIP file with the stuff interesting to have on the CI as well (current state of viewer & editor, benchmarks).

The CI would be expected to unzip the created ZIP files and deploy them for the result page as it seems useful:
* benchmarks as before
* localeditor as before
* viewer with the given example documents in viewer/docs, one link on the result page per document (should get also ODP and ODS possibly)
* the API dox from the webodf.js.zip
* the XPI, so people could download and test to install it

If that works, future steps would be to add also the unit test page for the browsers. So everyone can see if their browsers other than qtjsruntime also are still fine with the unit tests.

**Update2:**

There is still the new build target "webodf.js.zip-product" as described above.

The CI is now expected to call this:
    make # still is needed once, still needs to be fixed
    make products # or: make webodf.js.zip-product firefoxextension-product ? are the released products
    make viewerjsdir-product # to simulate what ViewerJS build calls
    make ci-products # all things special for CI, see comments at target definition

Once this is all done and worked without a problem, CI would see
* webodf.js-x.y.z.zip (from webodf.js.zip-product)
* firefox-extension-odfviewer-x.y.z.xpi (from firefoxextension-product)
* viewer/ (from viewerjsdir-product)
* programs/benchmark/ (from ci-products)
* programs/editor/ (from ci-products)

To allow some human testing, the CI build result page would offer
as before
* loading programs/benchmark/index.html
* loading programs/editor/localeditor.html

and new
* browsing the API dox HTML pages from webodf.js-x.y.z.zip
* downloading and installing the firefox-extension-odfviewer-x.y.z.xpi
* loading viewer/#docs/visual-tests.fodt and viewer/#docs/welcome.odt

Further documents for testing with viewer/ might be nice to have. Also do I hope we can add the tests/ soon as well for manual testing in whatever browsers (next step: using karma)

**Update3:**

PR refined now to this:
* new build target "product-library", which is creates a ZIP file with
  * webodf.js
  * webodf-debug.js
  * API dox
  * License
  * (README yet to be done)

* all targets which create end-products have a name prefixed with "product-", to make them stand out and be useful on tab-completion after `make`
* Marked the "zip" target as outdated and to be removed

CI will want to build using these build targets:
* product-library
* product-firefoxextension
* product-viewerjsdir
* editor-compiled.js-target (will be replaced on upcoming editor component)
* html-benchmark
* (browser tests yet to be done)