Make PyOperations execute on the main thread by default.
Add a flag to operations to indicate if they are thread-safe, and use this flag run-time in qtOperationLauncher, instead of making it a compile-time decision with SINGLE_THREAD define. PyOperation sets threadSafe to false by default, since the GIL is causing deadlocks or PyGILState_Check() failures in debug.
Fixed a few doc comments too.