[safety-maintenance] Automatic modification detection (change in GSN)

Solves following issue: #3744

Add to the maintenance status also "modified".

When a GSN node undergoes a change,
the referencing GSN node is to be annotated as “modified”.

(from redmine: issue id 3744, created on 2019-06-04)

• Relations:
  • parent #3009

Also,some bugs regarding the propagation of impacted elements were also fixed. Propagation of impact is now available also for modified elements.