Fix crash of test client

No wl_data_device_manager created in Mock compositor

Change-Id: If80f71165c28678f296bc18f0fea572dcb1a2f77
Reviewed-by: Giulio Camuffo <giulio.camuffo@jollamobile.com>
Destroy the QScreen when its wl_output is removed

Change-Id: I4532d05cca44c88fe38c14eacbe90c190f272b2c
Reviewed-by: Robin Burchell <robin.burchell@viroteck.net>
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
DataOffer not invalidated when client loses keyboard focus

The data_offer object should be invalidated when client loses keyboard focus.
Otherwise in following scenario, it will become zombie object: start app1 ->
copy text -> start app2 -> paste text -> close app1 -> paste again in app2 ->
seg fault in qtwayland.

The root cause is that when app2 takes focus the first time, data_device.data_offer
event was sent to it from DataDevice::setFocus. When app1 is closed, the data source
reference in data offer becomes invalid. so when trying to paste again in app2,
segmentation faults

Change-Id: I16a584e80fddaadd269b00cdf39eb405dd95b622
Task-number: QTBUG-41005
Reviewed-by: Giulio Camuffo <giulio.camuffo@jollamobile.com>