Undo / redo log visualization

Tentative UI: a button near undo/redo buttons. Click on this button opens a panel that has a scrollable vertical list displaying entries of undo/redo log. Entry corresponding to the current state should be highlighted. Left click on an entry undoes or redoes back / forward to the corresponding state.

Entries:

* Select ${entity}
* Change component ${component_type} of ${entity}
* Add component ${component_type} to ${entity}
* Remove component ${component_type} from ${entity}
* Add child ${entity} to ${parent_entity}
* Set ${entity} parent to ${parent_entity}
* Delete ${entity} with descendants

Would be great to apply accent colors to identifiers such as ${entity}, ${component_type} etc..