Windows: Implement automatic mouse capture in QPA.

Windows: Implement automatic mouse capture in QPA.

Qt expects the mouse to be captured on any button press
until release.

Remove flags to store capture, use WinAPI GetCapture() instead.
Remove setMouseGrabEnabled_sys(), streamline code.

Replacement for the reverted change
6b5bbc531b30d8ece25425e39843c6ae1af1d045 for QTBUG-25977.

Task-number: QTBUG-27132
Task-number: QTBUG-27039
Task-number: QTBUG-25977
Task-number: QTBUG-26962
Change-Id: If86428eabfadcafd16da10f134a419f833185272
Reviewed-by: Qt Doc Bot <qt_docbot@qt-project.org>
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@digia.com>
Reviewed-by: Jørgen Lind <jorgen.lind@nokia.com>