#7 sub-models of MPD documents cannot be re-ordered in file-contents hierarchy

[Ben Supnik]

Bug: MPD sub-models can not be re-ordered in current trunk code.
Steps to reproduce:
1. Create a new document.
2. Create a second MPD sub-model.
3. Drag the MPD sub-models in the file-contents hierarchy to re-order them.
Expected behavior:
- the slots before another MPD sub-model are valid drop targets for other MPD sub-models
- on drop, order of sub-models changes
Actual behavior: the drop is not allowed, and an exception is shown in the console.

Regressions: this change was introduced with the LSynth changes.