Add a simple 'Inspector' window.

[Kleptine]

I'd like to have a flexible side panel for showing more debugging information, historical values, etc.

It should:

• Have some logic to open / close it and remember that state across editor domain reload, etc.
• Be general enough so that we can mount it into a separate window.
• Support multiple inspectors open (ie. showing two node histories at once, etc)

It might make sense to just make this a separate window, because you could rearrange it, and would get open/close saving for free, etc. Not sure. It'd be harder to automatically open it though, the way you could with a side panel.

[vertxxyz]

Note that the inspector has been merged, but the tabular history view is its own beast. See PR #83