Implement INCR property mechanism for large data transfers [XCB]

Implement INCR property mechanism according to the icccm specification.

Change-Id: Ic8f85b71cab825d70ee1b61f29acd09fa4c3e642
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Fix timing issue in incremental data retrieval mechanism

Make sure that we are not reading wrong PropertyNotify events.

From icccm:

Requestors may receive a property of type INCR in response to any
target that results in selection data. This indicates that the owner
will send the actual data incrementally.

The selection requestor:
- Waits for the SelectionNotify event.
- Loops:
  + Retrieving data using GetProperty with the delete argument True.
  + Waiting for a PropertyNotify with the state argument NewValue.
- Waits until the property named by the PropertyNotify event is zero-length.
- Deletes the zero-length property.

The issue with the current approach was that after receiving INCR we
simply fetched the first PropertyNotify event from the queue, where
timestamp indicates that it was generated before INCR, which is not
what need. We need PropertyNotify events with a timestamp older than
the one on INCR.

Task-number: QTBUG-32045
Change-Id: I3b9a006f7d346f67c51e1a296d67a9dc89efadb1
Reviewed-by: Lars Knoll <lars.knoll@digia.com>