Manage mouse event delivery to popups and qvirtualkeyboard

QGuiApplication delivers mouse events to active popups (e.g. in processMouseEvent().
A click outside the active popup may close it.
That logic interferes with qtvirtualkeyboard, which has lead to e.g. QTBUG-126752.
https://codereview.qt-project.org/c/qt/qtbase/+/583116 provided a very specific fix: The list view of a completer will forward mouse event outside its geometry to a virtual keyboard, if one exists and is visible.

Instead of class-specific implementations, a general solution should be provided on QGuiApplication level. An approach would be:

• make qtvirtualkeyboard register its window on QGuiApplicationPrivate.
• allow a popup to register, whether specific mouse events should be forwarded to a virtual keyboard.