Add plugin capability to manage a secondary top node, to be used by upcoming interstage decouplers and existing interstage petal adapter. Would allow for sinking an engine cluster partway.

Thinking more on it, this needs to be a general capability for any part implementing engine mounts; need to snap the new bottom-facing node to the 'fairingOffset' position for the mount. -Should- be easy enough, as the node-adjust code is stable and robust at this point.

Will need to add a button/etc to enable/disable this node, or it might create problems with stock node-snapping implementation.

Implementation:

• Using SSTUSelectableNode (updated for single-node management)
• Using a basic editor-vessel-changed callback for coms between selectable node and other modules'
  • SSTUSelectableNode fires an editor-vessel-changed event whenever it toggles the node
  • SSTUModularXXX acts on this event and always updates attach-nodes whenever this even is recv'd.

TODO:

• Testing/validation.

DONE:

• Update module code for:
  • SSTUModularFuelTank
  • SSTUCustomUpperStage
  • SSTUEngineCluster
• Add SSTUSelectableNode module to config file for:
  • SSTUModularFuelTank
  • SSTUCustomUpperStage
  • SSTUEngineClusters
• Add attach node to config file for:
  • SSTUModularFuelTank
  • SSTUCustomUpperStage
  • SSTUEngineClusters
• Fix up SSTUSelectableNode config for LC-ENG parts (ASCE/DESE; 6 parts total)