Make Alt-dragging "additive" with XZ plane dragging, rather than replacing it

[Calinou]

Right now, when you select a shape and move it, it only moves along the XZ plane until you hold Alt. This is good behavior, but when you hold Alt, the previous XZ movement you've done is replaced by the Y movement. Instead, I expect all future mouse movement to only affect the Y axis until I release Alt, while keeping the XZ plane movement intact for the current move operation.

This allows moving an object along 3 axes with a single mouse drag, while still benefiting from the default behavior (XZ plane constraint). See godotengine/godot-proposals#3006 for a video that shows the behavior I'd like.

This proposal applies to all kinds of dragging (shape, vertex, edge, face).

[blackears]

I'm not sure this is such a good idea. Incremental movements can cause rounding errors to build up and give you results that you do not want. At least I think it would be a good idea to have a tool setting that lets users switch this option on and off. It's also going to have to wait until after I'm able to make the blocks no longer dependent on the CyclopsBlocks node.

[blackears]

I've added a drawing gizmo to the move tool which should make this suggestion no longer required.