Handle secondary history stack for specific tools

[sprocketc]

There are cases where we want to maintain a history of actions for a specific tool, but we don't want to add this state to our main history.

For instance, when we are trying to select multiple elements, being able to undo a selection would allow us to keep all of the previous actions instead of deselecting all elements and starting over.

Another example would be when we are drawing a polyline. We need to be able to go back and remove the added points step by step.

The tricky part would be to find a way to create a secondary action stack for such cases.
Although adding those action to our main history tree would work, it would create am extremely verbose history.
It would also make the history navigation harder and would increase the file size / memory footprint (see #26).