This patch does the following :
- as a vout window provider, make sure skins functions run by vout thread(s)
  don't issue any calls to the GUI. (skins on Linux runs fine at last !)
- prepare support for SET_VOUT_SET_FULLSCREEN new control
- implement hotkeys (no longer available at the vout level)
- modify a show() method not consistent across platform
- cleanup (dead code)

skins2: avoid deadlock when audio-visual is used (accounts for vlc often but not always hanging in this case)

Audio-visual holds the audio_mixer lock during its initialization phase which includes initialization of the vout.
On the other hand, skins polls every 10ms to check change in the audio volume (this also requires holding the audio_mixer lock)
This leads to a frequent deadlock at the skins level.

The patch stops polling w.r.t volume and rather reacts on change signalled by the core via a callback. This makes deadlock occurence almost nil.

A long term solution could be that 1/ audio_visual doesn't keep on holding the lock during vout initialisation and 2/ skins stops polling every 10ms for a whole bunch of things (rather use callbacks as qt4 does)

This problem appeared in vlc1.1.
setting volume from core or from the user slider must be differentiated, as sending back a zero value (mute) tells the core to really set value to zero (thus erasing the saved volume)

this patch reuses what is already done for position slider (similar issue)

This does not properly account check changes, so this is usual that this breaks.

This is ugly, until jb finds "a clever solution."

Fix for [1f579327] for Windows

[62bd7af8] and [182692ca]

This reverts commit e70077bf.
This was completely broken (NULL not defined).

This reverts commit 4bfba26a.
This broke strict C aliasing rules.