Need to support redo/undo

[TheDuckCow]

Currently, the addon does not very effectively support undo/redo operations. Meaning the user may add a roadpoint or change the number of lanes, and then if they hit control-z, instead of undoing that operation, it'll undo some other previously registered operation (without actually changing the thing intended). This applies to essentially any operation we run in the editor. To name a few that need to respect undo/redo:

• [x] Add next/prior roadpoint
• [x] Changing magnitude in 3d widget
• [x] Changing number of lanes via panel
• [x] Changing number of lanes via 3d widget
• [x] Moving roadpoint via blue collider (already working)
• [x] Changing any settings via "export vars" (already working)
• [ ] Adding of road network (and the auto-generate points/segments nodes)
• [x] Trigger update of LaneSegments after RoadPoint transform
• [ ] Trigger update of LaneSegments after RoadPoint transform undo
• [x] Updates to the RoadNetwork settings such as change material or density
• [x] Undo of adding 2x2 road from create panel

See documentation here: https://docs.godotengine.org/en/stable/classes/class_undoredo.html

[TheDuckCow]

Bret tried working on this for the addition of adding a road point, the undo point wasn't working well and had some side consequences which we want to investigate.

[TheDuckCow]

All but two are now addressed. Officially closing now, remaining two going into backlog.

Edit: Posted the extracted out commands here, including some additional searching that I wish I had done sometime earlier: https://github.com/TheDuckCow/godot-road-generator/issues/84