Drag and drop state not updated on keyboard modifiers changes

The drag and drop visual state is not updated on keyboard modifiers changes.

This lead to temporary incorrect mouse cursor until the mouse gets updated.

This makes it very non intuitive while other frameworks propose this behavior already.

The reason that the keyboard modifiers are not handled is because the key press/release events are filtered out (except fixed case to leave drag operation by escape key) @ https://code.woboq.org/qt5/qtbase/src/gui/kernel/qsimpledrag.cpp.html#147
Here we could update the internal application button_modifiers and send a mouse move event (while still filtering out key press/release events) so that the drag state can be updated correctly while pressing/releasing keyboard modifiers.