Mark multiple selections

[andeplane]

As of 117ee99, we can hover selected atoms in the menu which will highlight the selected atoms by making the other atoms darker. I tested letting these boxes in the GUI be clickable so we can... in a sense select selected atoms so this highlight render mode (darker non-selected atoms) lives on even when not hovering.

This didn't work out too well since you might get problems if you move the camera (clicking) and select another atom by accident.