Don't start in fullscreen

We never actually started in fullscreen with the GTK GUI, but now this
setting is actually respected so needs to be set to False. :)
Make it possible to tell modRana to start in fullscreen

ModRana already starts in fullscreen if the given platform requires it,
but there are some cases (such as for example the N900) where some users
might prefer the default behavior of starting minimized while others might
want modRana to start in fullscreen.

So add two options for telling modRana to start in fullscreen:
* option in Options->UI->GUI->"Start in fullscreen"
* --fullscreen CLI option