Add a make target for tests

We are running the existing test_mime.py. All the other tests which
are not unit tests but more examples of code one can run interactively
are moved to the example directory.
Add tests for bundlebuilder
Add test for the build command
Add test for the genpot command
Add a SUGAR_ACTIVITIES_PATH environment variable

Useful for tests but perhaps for other stuff too.
Add test for dev command
Some initial infrastructure for UI tests

The uitree module exposes the at-spi tree. We can use it to do
functional tests of the UI, by checking if the expected
widgets exists, clicking them etc.

A simple example of how this can be used is in the test, which
runs a window and check that it has the expected button.
Bind xdgmime in SugarExt

And use it in the sugar3.mime modules. This allows
to get rid of the pygtk generated sugarbase module,
along with the wrapping code.

Also add more cases to test_time to make sure
everything is still working.