Add an setUnloadOnDestroy() function to QLibrary

Since QLibrary does not automatically unload the library (i.e. by calling unload()) when the QLibrary is destroyed then it is possible that this is forgotten and therefore the library is still loaded in memory. By adding a setUnloadOnDestroy() function it would mean that it could automatically get unload() called when it is destroyed.

In Qt 6 this could be change to default to be on automatically if so desired.