skins2: improve callback system

skins2: improve callback system

This patch is mainly intended to work out the async queue getting
bigger and bigger when skins cannot keep up with command processing.
Since flushing the queue is an infinite loop, this ends up in vlc
hanging (frozen GUI). This happens when skins are complex (many controls,
many timers, animated images, scrolling text, ...)

With this patch, new commands of a given type can now replace pending
commands not yet processed when that makes sense (e.g position, volume, ...).
The position is really the one responsible for overflooding the queue.