Promoted plug UI improvements

This pull request addresses #264, allowing custom UIs registered for plugs within a Box to be successfully transferred to any promoted plugs on the outside of the Box.

Note that this change is backwards incompatible for any files containing CompoundDataPlugs with dynamic children. I think backwards compatibility could be provided (without the proper UI support) but given the early stages of the project, I'd rather just forge ahead. It should be fairly straightforward to fix up any affected .gfr files with a little bit of text editing.