Update the github book editor to use the refactored code

There are still several outstanding issues and should be probably be merged once loading and saving books works again since the current branch works.

Changes in this should be compared to the `master` branch even though the request is to the `github` branch.

So far, this PR does the following:

* `[X]` load a `META-INF/container.xml`
* `[X]` load the OPF File(s)
* `[X]` using the manifest, instantiate a bunch of `XhtmlModel`s
* `[X]` parse the special navigation file and
* `[X]` change the title of `XhtmlModel`s based on the ToC file