🍒WidgetsAskUserActionHandler: create message dialog in the main thread

In Qt, the GUI runs in its own thread and must be accessed from the
main thread. If QWidget is created in a non-main thread, we can use
the QMetaObject::invokeMethod() method to create it in the main thread.
This fixes a crash in plasmashell where there is no active QWidget
window.

BUG: 465454
FIXED-IN: 5.106

(cherry picked from commit 8270470d)