qt: get rid of WindowResizer

- Now that size change can be requested from the UI at most
  once per UI frame rendered, WindowResizer has become less
  relevant.
- !1978 (vout: Use a fair display lock and interrupt wait)
  already reduced the waiting time considerably.

Since the UI uses VSync (except CompositorX11), it should
be fine to have blocking stuff that only take couple of
milliseconds to execute.