gdk: activate window on keyboard grabs

In 01455399 ("gdk: do not deactivate surface on keyboard grabs"), we made gdk avoid deactivating surfaces when another application takes a keyboard grab, by using has_focus_window instead of has_focus. That however broke activating surfaces when the gdk application acquired a grab itself, in which case has_focus_window is false but has_focus is true.

We thus actually need to use both: surfaces should be activated either because we have normal keyboard focus, or because we grabbed the keyboard.

Fixes #85

(cherry picked from commit 3287ac96)