Inspector improvements

[arya-s]

Covers the following points of #28

• I want a button to collapse all expanded entries in the inspector
• I want selected entities in the inspector tree to be scrolled into view if they are far down the tree
• I want the inspector controls to be fixed to the top so that I don't have to scroll a long tree back up to get to the controls
• I want the inspector tree entries to expand by clicking on a single element instead of being able to "fall through" holes in between (e.g. there is a gap between ▶ and the label of the entry)

[suchipi]

This is so awesome. Thank you, @arya-s!