Dragging elements from sidebar

[AlecAivazis]

This issue should act as the central location for discussion on dragging new elements from the sidebar.

In the current implementation (as of 5/7/2019) no mutations are sent to the API until the user stops dragging the shadow and drops it on the diagram. At that point, the client selects the element that is created allowing the user to customize it.

[AlecAivazis]

@aivazis do you want the factory shadow to show just the central diamond or the compact view? The original design shows the diamond, but I think we decided to show the full factory so the user could drop it with confidence they aren't disturbing their diagram. Is that correct?

[aivazis]

i think the compact view is preferable

[AlecAivazis]

@aivazis I spent some time today/yesterday exploring whether we needed separate types for the shadows. While it does overload the semantics of Factory, Im pretty sure the fields are almost entirely the same so i think the shortest path (and possibly even the "right" one) is to just have a single type that represents both FactoryPrototypes and FactoryInstances