[Move Elements] Better handle move of parts and components across projects

Automatic move of parts as described in https://bugs.polarsys.org/show_bug.cgi?id=1904 is not satisfying. Sometimes parts are referenced by other elements so we would have to auto-move these elements too, which isn't possible to do in an automatic way. User-interaction is needed: Rather than auto-move the part, we add a 'synthetic' backreference from a component to its parts, so that when the user adds a component to the stage, a backref to its parts forces the user to also add the representing parts to the stage. From that point on, the transfer UI could auto-propose a new parent for the part as soon as the user chooses a parent for the component and vice versa.

ECLIPSE-554409 POLARSYS-2347 @fdo 2019-01-21

cp-api / capella

[Move Elements] Better handle move of parts and components across projects #431