Refactor UI code (OMG SO BEAUTIFUL NOW)

OMG SO BEAUTIFUL NOW I CANNOT RECOGNIZE MA CODE

Let's see:

* Some widgets are out now, some widgets are in now.
  MainArea is deleted: it's actually better and it requires
  less linkage to add all the widgets on MainWindow.
  Some widgets that used to extend packing widgets such as
  ScrolledWindow or Paned, now extend the actual widget,
  packing is done in MainWindow.

* PageList and NotebookList (renamed to singular) now have
  PageStore and NotebookStore models associated to them
  and possibly in a future a custom cell renderer.

* Many fields are read-only public. Even MelibeaApplication
  is now using singleton pattern. This lets some widgets
  access the UI tree.

Still need some refactor on Storage module. Then I'll
start working on actual business logic of the application.