Add text items to children list last so that they show up on top of coll...

Labels are getting drawn before collections, and can get hidden. I moved the code adding text items to self.children to later to fix this. This may or may not be mooted (is that a word?) by the upcoming javascript refactoring, but I think it is worth noting.

The current tests dont' cover this issue, but my test uses networkx which probably shouldn't be required. Let me know if this is something you want to pull in and if you'd like a test to go with it.