This adds a tooltip to all the slider properties showing their
current value. Previously there was no indication of what
value they had. Also rename the SprayTool property 'thickness' to
'size' like BrushTool calls it.
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.
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. :^)
