skins2: implement relative positioning

skins2: implement relative positioning

So far in skins2, all x, y, width and height were given in pixels, and
no reference to the screen resolution could be made.

This patch implements the following :
- all x, y, width, height can be given in pixels or in % of their container.
  For windows, the container is the screen. For widgets, it can be the
  layout or a panel.
- Five new tags eases up positioning
  - position : relative position given as "Center", "West", "NorthWest", ...
  - x/y-margin : margins to take into account (in px or %) wrt the borders.
  - x/y-offset : additional offset to apply (in px or %).
- the image control can now accept a width and a height parameters.