Prototype user interface

Some toolbar elements may still be required later, but this is
the user interface of the application. It's kinda dirty, needs
some refactoring and moving widgets now and changing visibility
so that widgets can communicate with other widgets.

Base layout elements:

* MainWindow: main Melibea window. Contains all other elemnets.
* Toolbar: primary toolbar where the user can give commands.
* StatusBar: status bar. Contains a secondary toolbar with
  elements related to the notebooks.
* StatusToolbar: status bar's toolbar.
* MainArea: main application area.
* NotebookList: list registered notebooks.
* PagesList: list registered pages in the selected notebook.
* PageEditor: editor.

All wigdets but PageEditor are contained in the same file.
PageEditor is on a different file because it's expected to have
a buffer and a controller. Some other widgets may move to a new
file if they grow so much and if they have a lot of associated
clases to them.