nitpicker: Domain-specific coordinate constraints

This patch introduces a way to tweak the coordinate systems per
domain. The 'origin' attribute denotes the origin of the coordinate
system. Valid values are "left_top", "right_top", "left_bottom",
"right_bottom", and "pointer". Furthermore, the screen dimensions as
reported to the nitpicker client can be tweaked per domain using the
'width' and 'height' attributes. If the specified value is positive,
it is taken as literal boundary. If the value is negative, the size
if deducted by the specified amount from the physical screen area.