Allow the user to Undo / Redo changes #24

What?

The user should be able to under / redo changes with ctrl+z and ctrl+y or using visual buttons (in the menu for example). I imagine there would be some sort of stack implementation keeping track of each change the user makes and the state of the canvas.

Decisions that need to be made

• Important for when we add layers, should changes to the canvas state be saved or only the grid (undo creating a layer for example)
• How long should this undo/redo history last
• Should the change buffer be persistent between pg saves?

This will be both an investigative and feature creation task, and this may need to be pushed until most of the other work is done depending on the decisions above. That decision should be made early though