Add 'Quit' menu item, and fix invisible "File" on gtk2

I didn't even notice that the "File" part of the file menu no longer
showed up, since the keyboard accelerator for ^S worked fine..  But
apparently there's no default label associated with GTK_STOCK_FILE in
gtk2, so the "File" text went away with the conversion to GtkUIManager
in commit 4d62478e14fe ("Use the newer GtkUIManager for menu creation.")

The addition of a Quit menu entry with the associated keyboard
accelerator also makes ^Q "just work".

Of course, if we actually tracked dirty state etc, we could perhaps ask
the user whether they wanted to save or something.  But I'm not exactly
famous for my GUI chops, so ..

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>