Add menu entry for freeing unused DLLs

The Windows COM runtime uses delay-unloading of unused COM DLLs, with a default
delay of 10min. The new menu entry enables immediate unload of unused COM DLLs,
thus avoiding the need to wait for 10min. This makes it easier to investigate
DLL lifetime issues in ActiveX controls that would otherwise require a 10min
wait after the control have been closed.

Reference: https://msdn.microsoft.com/en-us/library/windows/desktop/ms678413.aspx



Change-Id: I48ea7748cc3d976a912f050ccb54c2be4a86486e
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>