Undo & Redo

#### Undo Button
Using this, the user should be able to revert their actions one step at a time. It could be done by making a stack and inserting each of the operations into it. If pressed once, the topmost element will pop out and will be undone.

_It could also have a drop-down menu which will show the list of the previous actions done by the user, so that they may be able to undo them all in one go._

#### Redo Button
Using this, the user should be able to redo the undone actions. It could be done by making a stack and inserting the elements that popped out of the undo stack. The redo stack will be emptied as soon as a new operation (other than undo and redo) is done. If pressed the redo button once, it will pop out its topmost element and it will be redone.

_It could also have a drop-down menu which will show the list of the previous actions undone by the user, so that they may be able to redo them all in one go._

The issue names no files, tests, or entry points. Start by locating the existing user-action and editor interface code, then determine how undo and redo should integrate with it. Done means users can undo and redo actions one step at a time, with new operations clearing the redo history; dropdown behavior remains an optional scope choice.

Written by the indexing model from the issue text.