gui: generate the glade file with glade-3.18

No code changes.

Signed-off-by: Jakub Filak <jfilak@redhat.com>
gui: simplify the event selector page

Signed-off-by: Jakub Filak <jfilak@redhat.com>
gui: conver report-gtk to GtkApplication

- improves compatibility with GNOME
- allows us to move "Preferences" to the application menu

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