Signed-off-by: Rafaël Carré <rafael.carre@gmail.com>

vlc_clone_detach() is less flexible but we never used that lost
flexibilty. However, it will simplify a later Windows fix as well as
safely exposing detached threads to plugins.

It might be a good idea to do the same on other platforms

Ref #3804

This rework includes:
   - activation with mouse move
   - transparency and fading out supported if the WM allows it
   - fsc no longer a child window
     (this latter point should work out the refresh problems
      often mentioned on Vista and Win7)

The possibility to trigger callbacks when changing preferences was removed
 lately. So this code is now useless.

It cannot work properly due to SIGBUS when a file gets truncated.
In fact, this plugin had been disabled by default for as long as it had
been in VLC.

Based on keep-alive RTSP requests

CommonDisplay() must be called at least once to ensure that the window
content is shown.