app: commit ongoing tool operations on tool change instead of cancelling

On tool change, we used to simply halt tools before switching to the
new one, which meant losing ongoing live-previewed tool changes, like
transforms, warps and color corrections. This change makes them being
applied to the image instead before switching to the new tool:

Add enum value GIMP_TOOL_ACTION_COMMIT that is passed to
GimpTool::control() before tool switching. Handle the new enum value
in all tools, and actually commit the previewed stuff. This changes
the behavior of GimpCageTool, GimpImageMapTool, GimpTransformTool and
GimpWarpTool.