Add placeholder feature to bridge module ("No Signal" anyone?)

Add placeholder feature to bridge module ("No Signal" anyone?)

This commit allows using the bridge-out and bridge-in module to
configure a placeholder stream. This placeholder stream replaces the
input stream if it dies for more than place-holder ms.

Example configuration:
 # The input
 new test broadcast enabled
 setup test input /home/antoine/media/wall-e/Sample.avi
 setup test output #duplicate{dst=bridge-out,select=video,dst=bridge-out,select=audio}

 # The placeholder
 new master broadcast enabled
 setup master input /home/antoine/media/wall-e/wall-e.avi
 setup master output #bridge-in{placeholder,placeholder-delay=5000}:display

 # control test play
 control master play

Placeholder and input should have the same format (aspect ratio, dimension,
codec, ...). Connect to VLC's telnet interface and issue some
"control test play" and "control test stop" commands to simulate input loss.
The placeholder stream should kick in after 5000 ms (5 s) of input loss.