Partially revert commit c73710e410,
avoiding updating tool widgets unconditionally on tool resume in
GimpDrawTool -- it's too expensive in general.
Instead, handle display-shell changes in GimpToolWidget, by adding
GimpToolWidget::update_on_{scale,scroll,rotate} flags, which
subclasses can use to request an update on any of these events.
Set the flags as necessary for the affected widgets.
Add a new GimpToolFocus tool widget, which defines a focus region,
consisting of an inner limit, an outer limit, and a transition
midpoint between them. The widget allows controlling the limits
and the midpoint, and moving, scaling, and rotating the region.
Ell