Maliit: Fix enter on VKB not resulting in text input field acceptance

Maliit: Fix enter on VKB not resulting in text input field acceptance

Pressing enter on the VKB should result in the emission of the accepted()
for text input fields in QML or the returnPressed() signal in QLineEdit.
In that case the Maliit input server calls our keyEvent method over DBus,
which this patch implements.

Change-Id: I46170c30315adaf21db0e232f53780ccaa67e4bf
Reviewed-by: Jan Arne Petersen <jpetersen@openismus.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@nokia.com>