Fix bugs in main window in handler of configuration updates

- related to #14 and #10

Signed-off-by: Jakub Filak <jfilak@redhat.com>
Allow only a single instance of gnome-abrt

- the second instance will be producing DBus error until the
https://bugzilla.gnome.org/show_bug.cgi?id=694803 bug is fixed

- closes #14

Signed-off-by: Jakub Filak <jfilak@redhat.com>