Fixed keys handling for multilayout environments.

There is a bug in GTK+ that does not allow capturing shortcuts using
letter keys on layouts other than "us". The bug is there for ages and
there is probably no hope that it will get fixed. This patch switches
shortcut handling method to GtkAccelGroup, which handles this case
correctly. Enjoy!

Signed-off-by: Christoph Lohmann <20h@r-36.net>