Reapply mouse event handling in QQuickWebViewFlickablePrivate.

This reapplies commit 0b789c6a785d5ef45c3f6f2fc0f8694772b3d392,
which had been in Qt 5.0.x and 5.1.x, but was not upstreamed
and hence overwritten when importing the new snapshot.

This basically reverts http://trac.webkit.org/changeset/136119.

Task-number: QTBUG-35097
Change-Id: I3e02c3feeb5193c0c06b7525469e208de1d6cafc
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>