Fix Windows mouse enter/leave event generation

Fix Windows mouse enter/leave event generation

QWidget::underMouse() did not report correct widget in cases where
mouse was grabbed by popup, which was especially disruptive in case
of QCompleter popup, as that wouldn't close anymore with off-popup
clicks.

Root problem was that mouse capture in Windows caused enter/leave
events for QWindows to be generated incorrectly.
QPlatformWindow documentation specifies that enter/leave events
should be sent independent of explicit mouse grabs and only automatic
mouse grabbing done when button is pressed should suppress
enter/leave events. Updated Windows mouse handling to conform to
this.

Task-number: QTBUG-27283
Change-Id: Iecf786a702f7d29e6026c42ff8ec4c9cbf1b6ac3
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>