application: Let only About and Quit in the menu.
UI: Use the full name for the window.
ui: Use CSS to set the background of the image widget darker, so it
doesn't look like wasted space. Also, try to conform to the HIG3.
ApplicationWindow: Use behavior instead of properties for the
children. The class is so much nicer now, and it actually uses OO.

Application: Use the new methods from ApplicationWindow instead of
accesing its properties directly.