Instance grouping warnings

[issa-tseng]

We have tools now to group shapes together into instances (#22), but in cases where novices are trying to use the tool or the training is not highly specific, it can be easy to overlook these tools and forget to perform the grouping. The goal here is to enable the entity vending the work (eg whoever is providing the application configuration) to declare certain expectations—for instance, that a hand polygon should always be part of some instance group containing a forearm and a.. whatever the other part of the arm is called.. polygon.

• configuration option to display warning when a polygon is expected to be part of an instance but isn't
  (only when not selected?)
• configuration option to display warning when some combination of objects is not an expected instance grouping
• configuration option to display warning when any polygon is not part of an instance