Fixed Mumble not remembering hidden dockwidgets

- Switch to custom mode if anything gets hidden/shown so
  auto-layouting code doesn't just unhide them on startup.
Abort drop when drag setting is invalid
Reduce probability for possible race until a real fix can be implemented

- p's target might be invalidated in the time between retrieving it and using it. https://mumble.hive.no/crashes/win32/1.2.4-145-g442b46e/2013-09-13T23:16%201.2.5%201.2.4-145-g442b46e%20Unknown.html seems to indicates that this happened at least once in Line 809 of AudioInput.cpp (mind the inlines).
- Should be fixed properly by using smart-pointers or removing this data interdependence between threads