mumble_exe: add user-visible alert dialogs for error states.
mumble_exe: do not add mumble.exe's parent directory to PATH.
build: add support for building against Qt 5 from the win32-static build env.
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
Re-implement UserEdit using Model/View paradigm to resolve various problems
with the old implementation.

- Added UserListModel.h/cpp as the backend for protobuf UserList editing
- Added UserListFilterProxyModel for filtering and sorting
- Re-implemented previous functionality in new environment