gdk: activate surface on keyboard grabs

gdk: activate surface on keyboard grabs

In 01455399e83a ("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.

This also renames HAS_FOCUS to APPEARS_FOCUSED to better reflect its
role.

Fixes #85