This adds a History view of your nbpresent content (e.g. slides and themes, not your notebook), constituting an initial stab at #74.

Here's a shot from the tests:

History goes back to your initial load from the server, the following activities are snapshotted/undoable:

• Move/Resize Region
• Nudge Region
• Load from Server
• Slide Layout
• New Region
• Set ${d.attr.key}
• Set ${d.attr.key}
• Set ${d.attr.key}
• Reordered Slides
• ${d.title()} Import
• Set Slide Theme
• Link Cell Part
• Unlink Cell Part
• Delete region
• Add Slide
• Remove Slide

There are some situations where a user could create an "unusable" history: the first time a cell's content is used, it is given an UUID. Going back in time will not unset that UUID or otherwise, but this seems like a pretty reasonable thing.

No history is ever culled (until you reload the browser) which might become an issue, as you can create some interesting timelines (which are always sorted by creation time). Often, behavior will be to remove "dead" branches, but in my local testing, I kind of liked being able to store multiple prior heads for looking at different approaches. Not to mention memory-leaky... worst-case scenario would be big images used as backgrounds, etc.

If you end up having to reload your page, then, you'll also lose history... but this seems not unreasonable.

I have also not added any keyboard shortcuts or additional buttons for undo/redo, as ctrl+z seems like a bad thing to take over.

@jbednar: is this starting to answer the mail on some of your concerns?