History by layer

[Iyusuf40]

This update limits history changes to affect only shapes, so undo/redo does not affect layers. Initially undo/redo could delete and re-add layers in addition to shapes.

PROS:

• We can now work on different layers, use undo/redo functionalities that affect only the currently selected layer, so other layers remain unaffected.

CONS:

• Initially we could delete a layer and still have it back by undoing that action. with this current change, deleting a layer means it can never be recorvered, unless you have a json backup somewhere.

[gniziemazity]

Let's use this version for a while and decide if it's better than the previous. Eventually I want to refactor the undo functionality so it just records the change and the way to undo the change.