Use abstract base class to specify converters protocol.

The protocol was already specified as a docstring, but formal description may be more convenient and useful.
Rename oxalis.converters.converter module to oxalis.converters.base
Make MainWindow a subclass of Gtk.ApplicationWindow
Split out StartPanelController from the StartPanel

StartPanel is now just a widget.
Move functionality between MainWindow and controllers

StartPanelController now handles site conversion, SiteWindowController
handles site loading. MainWindow on the other hand takes care of
setting up header bar and menu.