MAGN-4845 Cannot reopen dyn with Family Types node saved with daily build

http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-3832

Requires cherry-pick into:
- [ ] Master

For Review:
- [x] @Steell 

This is in response to #2595. The change introduced yesterday to add name indexing to drop-down properties was not properly hardened. Specifically, for the Family Types node, where we list the items as foo:bar, the strategy of parsing using the : delimiter fell apart as the index would be saved as 1:foo:bar and when parsing we would just get back 1:foo. The new method uses substring to return everything after the first :. 

In addition, I've added tests to make sure that this doesn't break again. EVER. Edge cases are selection index of -1, out of bounds selection index, and missing name during lookup.