frontend: replace knit frontend logic with frontend model

- dia to sml: remove is target from context; populate and use model's
  is target instead
- knit: remove persister, provider, etc; make use of frontend instead
- frontend: improve error reporting around extensions in registrar
- frontend: fix workflow bug when disabling model module
- sml: improve documentation of merger
- sml: merger and worfklow no longer distinguishes between target and
  reference models, making use of is_target instead
- sml: library models are no longer handled by workflow.
fix bug when saving dia and xml models