A "feather" value sets by how much the borders of the selection will be
smoothed, and a "mode" value sets how the newly selected region will
interact with an existing image selection (if any).
If you press "spacebar" while moving a selection, it will now move the
origin point of the selection; and if you press "control" it will move
it relatively to the center.
This patch moves the marching ants painting code to Selection and
unifies the timer mechanism so that all marching ants are synchronized
which looks neat. :^)
