WinRT: Fix TCP socket reads

All read calls are now pulled from an intermediate buffer which is
populated from the asynchronous callback (this was a TODO previously, and
was breaking downloads of large requests). As a side-benefit, the use of
only async callbacks ensures fewer first-chance exceptions appear in the
debug output.

Task-number: QTBUG-30196
Change-Id: I5653742d8d94934a4b4a4227298865d20518bc4c
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>
ANGLE WinRT: Call Trim() when application suspends

This is required by Windows Store Apps to pass certification.

Task-number: QTBUG-38481
Change-Id: I6dc00431ee5f6c7d4c64111ccc38f46483d3b9a8
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Maurice Kalinowski <maurice.kalinowski@digia.com>
WinRT: Handle back button as press or release

Earlier, only the back press was checked for acceptance. By also checking
the release event, this makes the backstepping behavior consistent with
Qt for Android, and fixes the expected behavior found in our demo
applications.

Task-number: QTBUG-35951
Change-Id: I9c2f18816b838d57713ba4dd3624e2f3f1ac40ac
Reviewed-by: Maurice Kalinowski <maurice.kalinowski@digia.com>
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>