Use (thick) postprocessing driven outlines for highlight selections

[Wumpf]

Currently, almost all our selection highlights in 2d/3d scenes are based on changing the color/brightness or size of respective objects.

This interferes with the user chosen size and color and is overall not very expressive.

Instead, we want to add outlines in different colors. This should be done in a way that is scalable with arbitrary objects and can (optional or always?) show outlines of (partially) hidden objects.

There is an excellent writeup on how to do good looking outlines with arbitrary thickness using jump flooding by Ben Golus:
https://bgolus.medium.com/the-quest-for-very-wide-outlines-ba82ed442cd9