skins2: extend xoffset and yoffset parameters

skins2: extend xoffset and yoffset parameters

Till now, these two parameters only had a meaning for geographical positioning.
As explained at http://forum.videolan.org/viewtopic.php?f=26&t=111572,
extending xoffset and yoffset for the general case would help a lot skin
developpers easily position an object of unknown size around a given point :
for instance,
    x="50%" xoffset="-50%" would center an object horizontally.
    x="100%" xoffset="-100%" would set an object to the East

This patch doesn't modify the existing behaviour. It just implements a new
feature for the case where it was a silent no op.

x,y, width, height percentages are computed against the parent container
xoffset and yoffset percentages are computed against the current object

TODO: this approach could be generalized to all objects (need to extend the dtd)