Clean up the various mouseup / mousedown behaviours in the editor

[maltenuhn]

Problem
Our handling of UI controls has regressed / become inconsistent in terms of what activates onMouseDown vs onClick / onMouseUp. Here is a list of the behaviours we should change.

We should also make sure that all buttons that activate on click get a hover state.

Navigator onMouseDown

• Navigator tabs (file browser etc)
• Navigator item
• eye and lock icons
• insert / change buttons (?)
• expand arrows for scenes
• expand arrows for items

Inspector onMouseDown

• Inspector tabs (Inspector / Comments / Settings etc)
• Expandable sections (when you click on the heading / expansion arrow) ie Styled, Image, Advanced, etc
• Any expansion indicator (including component prop tree view)

Inspector onClick

• [+] and [-] buttons eg for grid (should also highlight on hover)