Make sure timestamp is initialized before using it for seting selection owner.

Make sure timestamp is initialized before using it for seting selection owner.

Convention from icccm: Clients attempting to acquire a selection must set
the time value of the xcb_set_selection_owner request to the timestamp of
the event triggering the acquisition attempt, not to XCB_CURRENT_TIME. In
some cases it happened that timestamp was set to XCB_CURRENT_TIME.

A zero-length append to a property is a way to obtain a timestamp for this
purpose; the timestamp is in the corresponding XCB_PROPERTY_NOTIFY event.
We used to have this mechanism in 4.8, it was achieved by XWindowEvent.

AFAIK there isn't an equivalent for XWindowEvent in XCB. Therefore i had to
introduce a new mechanism in QXcbConnection - getTimestamp. This function
blocks until it receives the requested event.

Change-Id: Ide46a4fdd44cf026fdd17a79d3c4b17741d1b7d4
Task-number: QTBUG-26783
Reviewed-by: Uli Schlachter <psychon@znc.in>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>