First working GUI prototype

In the last two days I’ve written a working prototype of the main window that will compose NoteDown. It can be found in the git repository. Supported operations:

• Text insertion (well, no big issue here, GTK takes care of that);
• Bold/Italic/anything on selections (still no big issue, the Tutorial covers that);
• Bold/Italic/anything on current word (zim-style)
• Least astonishment behavior for Toggle-Buttons in the Toolbar