Make playground editor allow multiple Undo's

[Daniel-Shunom]

Currently, the playground only, for lack of a better word, undo's the most recent change in the editor, so pressing CTRL + Z multiple times does not undo changes beyond that single latest change in the editor.

To reproduce, simply make multiple repeated changes in the editor, and attempt to make multiple undo's.

[lpil]

Would be good to have, aye!