Create undo for element's settings

[akaDuality]

Probably fixed after #339

Undo operation is broken. Previously the app observed changes in the array of elements, but after reworking the Artboard format setter for elements isn't called more.

TODO

• Observe changes in element, add support for undo.
• Add UI-tests for that
  • Also test for trait changes and observing of new value, not the previous one
• Repeat for container and frame