src/aven.cc: Don't persist full screen mode between runs - it's not a
standard behaviour of desktop programs, and it's too easy to go into
full screen mode from the menu and then not be able to get out again
because you don't know the required key shortcut.
src/: When in full screen mode, moving the mouse to the top of the
screen now makes the menu bar appear (and moving the mouse down again
makes it disappear).  This provides a non-shortcut way out of full
screen mode, as well as making it easier to perform other operations
while in full screen mode.