This fixes #3626.

Without the shortcut, it is not possible to start the access_demux
for skins, as the full URL is set to file://<path>.vlt

Now we can read skins packages as in earlier versions:
 $ vlc your_skin.vlt

Ref #3630

Notably: do the difference between Unknown and Unsupported Elements so the bug reports are cleaner
do comment deprecated stuff

This saves one thread. Previously, the main thread was doing nothing.

This is more flexible than the current libvlc_InternalWait() which
requires a thread that does nothing for the whole lifetime of VLC.

This should fix VLC reporting an interface error if it is interrupted
early.

As the "descriptor" function is called with absolutely no libraries
loaded, even the built-in "require" function is not present at scan
time.
This closes #3453.

Note: this is a bit of a hack, but I prefer not to load any libs when
scanning the available extensions

Remove some useless debug, too

This reverts commit a9175056.

If the main interface does not contain any video, it has no business
with the video-on-top flag.

This presumptively fixes a probable race condition. For the umpteenth
time, never ever make any Qt4 UI call from the video output thread!!

The main interface went always-on-top when the video widget was created
but it failed to return to normal state when the video widget was
destroyed.

Unfortunately, according to the Qt4 documentation, changing the window
flags hides the window (until show() is called). So I guess there is no
way to toggle the on-top mode without the ugly blinking effect. Then,
try to minimize its occurences.

block_FifoWake does not work as I thought.

If false, the vout core will use a thread to dispatch key events.
It reimplement [cb72d26f] using a bit less code
and by adding support for multiple keys pressed in a row.
The thread is created only if needed.

This reverts commit cb72d26f.

This reverts commit a0c1d2cc.

This reverts commit 031dec33.

This reverts commit 4b435093.

We have the same bug in several outputs (SDL, AA, CACA), so lets make
the solution generic.

This is consistent with the current mouse filter, and how people tend
to interpret the horizontal and vertical words. Until now, VLC would
refer to the rotational axis direction rather than the direction of the
changed dimension.

Fixes: LB#568750