QMenu: Enable sloppy submenu mouse navigation

QMenu: Enable sloppy submenu mouse navigation

Since we're in the 21st century, we set QCommonStyle to return true
to the SH_Menu_SloppySubMenus style hint. This unlocks all the logic
already available in QMenu.

This is a backport of I134c87e348d98d1f46055e0bfef2b4a4a3d2993a
from qt/qtbase and includes some previous enhancements.

Task-number: QTBUG-20094
[ChangeLog][QtWidgets][QMenu] Enable sloppy submenu mouse navigation
Change-Id: If4141a3d849123850e40af92ac5aede9ed767bdf
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>