Implement toggle

[pburtchaell]

• Adds monitorState object to track the state of the monitor, e.g., if it is visible or not.
• Adds show() action creator to show monitor
• Adds hide() action creator to hide monitor
• Moves the padding from the monitor container to the monitor itself; having the padding on the container prevented the monitor from being fully hidden.
• Adds some help text at the top of the monitor to explain how to hide the monitor.

[gaearon]

Thank you!!

[gaearon]

I think we can probably generalize show and hide as something like setUIState and this.props.uiState so different monitors can use devTools.js but store different things there.

[pburtchaell]

Yeah, that would be a good idea. Perhaps something to mention on #3.