MUST-HAVE new PartDesign with Body and Datum features

[jrheinlaender]

This thread is dedicated to discussion of MUST-HAVE features in the new PartDesign branch with the Body feature and Datum planes/lines/points. The code can be found here:
https://github.com/jrheinlaender/FreeCA ... partdesign

Please use this thread only for MUST-HAVE features, there are other threads dedicated to bugs, optional features etc. etc.
I will keep a running summary of the status of the suggestions here in the first post.
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- Confine dependencies between Bodies to the same Part and provide an Interface system for external references
- DONE: Selection highlighting for datum planes
- Prevent the error message "This feature is referenced by other features..." whenever a feature is deleted from a Body. Maybe put something in PartDesignGui::ViewProvider::onDelete() ?
- DONE: Booleans with Bodies
- DONE: Allow dependencies between Bodies. On the Command level, limit them to inside the same Part and make a check for circular dependencies.
- DONE: In the same spirit, allow external geometry from other Bodies in the Sketcher
- DONE: Allow to move a Mirrored/Linear Pattern etc. feature to be changed into a MultiTransform feature