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.