+ Added the Samples Tree placeholder
* Renamed the Window title

It no longer says "Hello World". Also, I fixed the status text.
+ Added a placeholder timeline
+ Added a tapes page to the sources notebook

From now on, the Sources notebook shall be called the library notebook.
This is to avoid confusion with the Sources page.
* Renamed SourcesNotebook and snb to Library

SourcesNotebook was a confusing name, since I'm now using the term
Source for a raw asset, and indeed, there's a page in the notebook
called SourcesPage already.
* Renamed SourcesSizer to LibrarySizer

Renamed this as well (forgot it the first time :P)
+ Added a placeholder sample property panel
+ Added basic skeleton framework for drag&drop