1) If "support on build plate only" is enabled, the support columns are
   newly trimmed to not land on top of an object. However this may make
   the column too small to be stable.
2) Support enforcers newly take precedence over "supports on build plate only"
   and over "don't support bridges".
3) Some refactoring of the support generator code for clarity: Reduced
   some of the worst spagetti offenders.

Fixes Support generated even if support on build only activated #915
Fixes Bug: supports on build plate only #1340
Fixes Bottom interface layer is not generated , support on build plate only. (long open defect) #4199
Fixes option "supports on build plate only" does not work #3980

Fixes No support interface layers generated #1997
Fixes Feature Request: Option to combine results of 'support from build plate only' and 'support enforcers only' #2801
Fixes Support interface isn't generated: build plate only + blocked by model + support enforcer #3831
Fixes Support Enforcer don't create interface layers #5748
Fixes Support Enforcers Don't Have Top Loops/Raft #1870
Fixes Don't cancel support enforcers with "don't support bridges" #5105