gtkwindow: invoke class event handler instead of implementation

GtkWindow has a special case to handle certain events before the regular
event processing, to deal with GTK+ grabs somewhere else preventing
interactions with e.g. popovers or modal dialogs (see commit
36abb4b829).

The special case will run GtkWindow's own implementation of the event
handlers; this will break the case when a GtkWindow subclass overrides
an event handler.

To fix the issue, grab the correct event handler from the object class
instead.

[endlessm/eos-shell#3683]
Merge pull request #7 from endlessm/shell/3683

gtkwindow: invoke class event handler instead of implementation