Tightening of the update rect when firing the marching ants timer
unfortunately meant that only finalized selections would be redrawn,
and any new selection drawn outside of the update rect would not
update.
This tightens the update rects for EraseTool, BrushTool And the marching
ants update in Selection. Inflate Selection update rect by 10x10 to
avoid misalignment when zoomed out.
The Selection object now tracks whether there is an ongoing interactive
selection (originating from one of the selection tools). If so it makes
sure to pump the marching ants animation.
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. :^)
