Till now, moving a skins2 slider was only possible if the pointer didn't get
too far away from the control (hardcoded parameter of 40 pixels in skins2).
Though this feature can also be seen elsewhere (firefox slider does it too),
the result may not look so natural especially for things like equalizers.
Moreover, qt4 doesn't implement this constraint, and therefore true skins2
sliders and qt4 sliders obtained from the popup menus were behaving differently.

So better remove this limitation and make things consistent throughout vlc
(skins2 + qt4)