This is a Gimp transform tool I made.
You can add up to four handles. If you move a handle in transform mode, a transformation will be applied so that the other handles stay at their place.
The type of transformation depends on the number of handles:
1 Handle: Move
2 Handles: Rotate and scale (keep aspect ratio)
3 Handles: Shear and scale (don't keep aspect ratio)
4 Handles: Perspective transform