Undo last annotation shortcut (control-z)

**I propose to add Control-Z hotkey or `U` to undo last action for text projects.**

Undo shortcut would be very useful, because currently if you make a wrong annotation, you have to manually click on it with a mouse and then cancel it by clicking on cross button, which is slow and inconvenient. Most text editors support Control-Z to undo last action.

It is related to the frontend for text annotation project.