layer move: strokemap too + cursor & idler tweaks

layer move: strokemap too + cursor & idler tweaks

Add support for moving the layer's strokemap after the user finishes the
drag. Do it unobtrusively in the existing idletask processor.

Cursor fix: now that layer moves are more responsive, the four-way arrow
or closed fist cursor feels nicer.

Internally, there is now one idletask processor per StrokeShape, not a
single global instance. Shapes can now be forced to completion
independently of other shapes, which may be slightly faster when picking
if the shape of interest is near the top of the stack. The payoff from
using individual processor objects is far more certain when queueing
strokemap translate operations.