On Windows, Qt needs to return false in case it does not handle the "extra"
buttons (like WM_XBUTTONDOWN) which causes Windows to send the
corresponding WM_APPCOMMAND message (like APPCOMMAND_BROWSER_FORWARD).

Task-number: QTBUG-48117
Change-Id: I093cd2d8205a39c3a042dd8a0d19c7229003761f
Reviewed-by: Andy Shaw <andy.shaw@theqtcompany.com>