Improve documentation of mouse behavior with modifier keys

Improve documentation of mouse behavior with modifier keys.

It is suggested to add a table that is similar to the table in the documentation for QKeySequence, with normal MousePress + modifier behavior.

Also emphasize what QDropEvent::proposedAction() is, customer feels that this is unclear (it is depending on platform and modifiers).

Have more information in the docs regarding the different platforms preempting policies (which mouse/modifier events are caught by the system, and never reaches the application).