First draft of the metadata browseer

This is just the first draft and not everything is working right now.

Here is just a short list of things that I have to fix or change:
* Redundant code: more code could be shared with the file browser. I will address this as soon as both browsers are feature complete (from the M2 perspective).
* Naming: I'm still unhappy with some class names e.g. `ResourceAdapter`
* Some code from the `ResourceAdapter` and the old `RDFTreeItem` should be available on the API level.
* Switch to rendered view is not implemented.
* Switch to search is not implemented.
* Favorites are fixed and can't be selected.