Add DataStoreReference to DataObjectReference replace menu and vice-versa

[azeghers]

Is your feature request related to a problem? Please describe.

In its current state of the DataObjectReference replace menu only contains the collection marker.
It feels empty and out of place.

As discussed with @andreasgeier, it has been suggested to make it possible for a DataObjectReference to be switched to a DataStoreReference, and vice-versa.
This will ensure that the menu is always populated with something, at least until the DataObjectInputs/Outputs are handled.

[azeghers]

First draft :

@andreasgeier

[azeghers]

Second draft, with some help from Mr. Fromme :

@andreasgeier

[andreasgeier]

It looks like you use the top left corner as a reference point when switching between symbols of different sizes. I would try the center of the symbol, otherwise, it would be no longer aligned when it is part of a diagram and connected with lines to other elements.

[andreasgeier]

The current solution is related to camunda/camunda-modeler#593