skins2: add width/height parameters to slider

skins2: add width/height parameters to slider

These parameters will allow to create a slider which adapts to the display.
width/height parameters will be optional. If set, the initial slider will
be resized to meet these requirements.

A main use case will be the possibility to create a fullscreen timeslider
that automatically spans the whole screen whatever the screen resolution.
e.g syntax would be <Slider width="100%" ....>