Multi-window

[rcarver]

This is a major redesign of the system to properly support multi-window applications.

You can now install a separate undo stack via dependencies:

withDependencies {
  $0.installDefaultUndoStack(undoManager)
} operation: {
  // ...
}

Then, the registered UndoManager and stack are fully independent of others.

Registering the UndoManager can also be done later, via defaultUndoStack.set(..) as in the TCA integration.

The implementation simplified significantly by doing this; we used to do complex tracking of undo sequence numbers which has all been collapsed to a barrierID for each undoable set of operations.

The example app has been updated with this pattern and now each window maintains an independent undo history.