Model-view-view model for sequences

Currently play_sequence is one of the few remaining "big features" not yet split from the main engine class. It has a carefully timed timeouts for fades in particular, which is annoying to try to adjust. Some events have been introduced.

Separation of responsibilities:

Model:

Since sequences are so much about presentation, I'm not yet sure what should be in the model. Maybe keep track of which slide is being shown.

View model:

View model should keep track of fading the screen in and out. Model cares about which slide we are at, the view model knows the state of the fade.

View:

Sequence layer and current slide showing and hiding.

We can probably get rid of the setTimeout handling and just rely on events firing and listened to when appropriate.

Probably a prerequisite for #102 too.

Start at the play_sequence implementation in the main engine class and trace its fade timeouts, events, and slide state. Review the proposed model, view model, and view responsibilities, along with prerequisite issue #102. Done means the sequence behavior is separated into those responsibilities without relying on the existing setTimeout handling.

Written by the indexing model from the issue text.