Consider holding a unique_ptr to the Document Window *inside* melatonin_inspector.h

[sudara]

Pros:

• Simplifies installation instructions (applications just hold a normal member, no need for users to make_unique or define onClose)
• Could attach key listener attached to root app so ctrl/cmd-I constructs and opens the inspector

@FigBug Do you care at all? Mainly motivated for the key command, I can also just add on my app side.

[sudara]

Also might help do some work on the way to #56

[FigBug]

I'm not too picky how the api looks, as long as I can have my menu item that creates / destroys the inspector.