Add a Property Inspector

[blink1073]

Description

Add a property inspector that is used to display properties about a widget. Currently this will be used by the notebook cell tools, but can be used by any main area widget to display additional properties about the widget.

Typically a property inspector would be on the right hand side (like in Photoshop), but we will keep it on the left side for now, since there is currently nothing on the right side. Users can choose to move it to the right using the context menu, and implementers of the extension can put it on the right if desired.