app: add GimpTransformGridTool; derive most transform tools from it
While most of our transform tools use an interactive transform grid, and have similar behavior, the flip tool is an odd one out. The new "auto straighten" function of the measure tool introduces another tool that performs transformations, while not behaving like the rest of the transform tools. Factor out the parts of GimpTransformTool that handle user interaction into GimpTransformGridTool (with corresponding GimpTransformGridOptions, and GimpTransformGridToolUndo), and only leave the basic transform functionality and options in GimpTransformTool (and GimpTransformOptions). Derive all the transform tools (and transform-tool base classes) that previously derived from GimpTransformTool, from GimpTransformGridTool. The one exception is GimpFlipTool, which still derives from GimpTransformTool directly. The next commit will derive GimpMeasureTool from GimpTransformTool as well.
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