QPlatformInputContext now gets notified on changed focus and
has inputMethodAccepted() telling whether current focus object
accepts input method events.
Also adapted IBus plugin to use this. Key event filtering for
focused objects without input method support got fixed by the
change.
Change-Id: I6910aa6af2459d752a5763f0ae88fa8c34e5b165
Reviewed-by: Joona Petrell <joona.t.petrell@nokia.com>