Add an undo_desc field to GimpTransformToolClass, which subclasses
should set to the tool's default undo description.  Provide a
default implementation for the get_undo_desc() vfunc, which returns
(a copy of) undo_desc.  This simplifies transform tools that have a
static undo descrption, as well as provides a fallback when a
detailed undo description can't be generated (not currently
relevant, but will be used in the next commit).