Refactored Instrument actor visitors to make them more generic.

Re #5403
Added method for finding flat detector assemblies. Re #5403
Calculation of the rotations for banks. Re #5403
Spread detectors over the plane so they don't overlap. Re #5403
Calculate detectors visible sizes. Re #5403
Can show RectangularDetectors. Re #5403
Added support for assemblies of detectors. Re #5403
Removed unused code. re #5403
Inherited all GLActors from QObject to prevent strange crash. Re #5403
Merge branch 'feature/5403_side_by_side_unwrap_view' into develop