Add a timeline to CaDoodle

[madhephaestus]

Kacper asked for a feature where the history of all of the actions taken by the user are visible as a timeline. The events should have an image to indicate the step, and have a label with the type of operation. When a step in the timeline is clicked on, the model should be rewound to that point.

It would be nice to be able to modify the operation selected and insert the change instead of pruning the steps ahead of it. For example any op that simply modifies the existing objects without adding or subtracting any might be able to be modified in place, with the following steps applied afterwards.