Editor: Redesign the selection markers

I'm sure most of us know the CS has some issues, one of the chief among which is the selection markers that result when you select an object. Particularly, multiple objects:

I feel it is not an exaggeration to state that this makes not only the markers unusable but the entire scene impossible to work with in many cases with multiple selections. They take up more space than the table in every one of these images.

Specifically, the following should be resolved:

1. Each worldspace widget (render view) should have a single selection marker
2. The marker should be associated with the history of objects which you have selected (eg, jump to most recently selected when selection changes)
3. Depth testing needs disabled so the markers will always be visible above everything else (other things need done too, see upcoming MR)
4. Preferably, we use a real widget instead of the nasty generation code in object.cpp
5. Scaling should be applied according to camera distance and not simply the radius of the object which it's attached to (this makes cartoonishly larger markers in many cases)
6. The marker should be responsive to both axis locking and mouse highlighting, so you know when you're actually going to interact with it.