app: add GimpTransformToolClass::undo_desc field
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).
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