Crash happens if you do:
 - Play item
 - Activate vfilter
 - Stop item
 - Deactivate vfilter... BooM.
(cherry picked from commit c876401791144309d2427214d818a34198238358)

(cherry picked from commit c31d99fdf67fbc9060fb9f413d06b0acf4efc1b6)

Already available in the interface sprefs.
This reverts commit c1a144df.

We were already blending directly on top of the destination
picture. This simplifies the code and should prevent further
confusion.

(it was used by net_OpenDgram when no server is specified).

This reverts commit 9a5e5236.
Not intended for master.

SleepEx expects a duration in milli second.

Make sure that we don't add the briged elementary streams to the next
stream output chain element when in placeholder mode. Maybe I should
split normal bridge-in and placeholder mode because the code is
begining to be filled with quite a few if( p_sys->b_placeholder ).

Add default enabled option to force placeholder state toggling to
happen only on iframes. This removes artifacts at the expense of a
longer switch delay.

This Widget (base QLabel) is designed to show the cover art. It handles right-click, and show a context menu that has only one entry:

- Download cover art

When the user clicks it, vlc downloads the cover art (or finds it in the cache), and displays it when downloaded

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.

This commit adds an in-name property to bridge-out and a name property
to brige-in which allows selecting which bridge-out go to which bridge-in
modules. Default behvior is unchanged (everything goes to the same
bridge-in).