Ability to undo/redo multiple changes in one step

[denzo]

This PR introduces new functionality without breaking the existing one.

Currently the undo/redo stack consists of list of changes. Therefore it is not possible to make multiples changes and be able to undo those in one step. This is easy to demonstrate via using setProperties for example.

tm.setProperties({
  completedAt: new Date(),
  isCompleted: true
});

Calling tm.undo() will not undo both of the changes.

In order to achieve the above this PR changes the structure of how we store the changes.

The undo/redo stack now stores a collection of changeSets which is nothing more than an Array of changes.

It looks like this:

undoStack/redoStack = [
  [Record, Record],
  [Record],
  [Record, Record, Record]
];

The current functionality of the addon stays the same. However, when multiple changes need to be recorded as one the user of the addon needs to call .startTimeMachine prior to making the changes and .stopTimeMachine when all the changes are done.

tm.startTimeMachine();
tm.setProperties({
  completedAt: new Date(),
  isCompleted: true
});
tm.stopTimeMachine();

Calling tm.undo() will undo both of the changes.

[offirgolan]

@denzo there are a lot of changes here. Can you explain what's going on and why these changes are needed?

[denzo]

@offirgolan Apologies for the delayed response.

I've updated the description of this PR and added tests and explanation in the code.

I hope it made it a bit more clear. Let me know what you think and if this is something you would consider including as a feature of this awesome addon.