Enhancement: Allow drag and drop reordering of TVs in Template TV grid

This is the first attempt to solve the annoying "not-being-able-to-reorder-TVs-by-drag-and-drop"-problem existing since Revo was released (and that functionality existed in Evo), so far it seems to work quite well, but as I'm far from even little skilled with extJS, I don't know if my approach is even a valid one...

Related issues (maybe there are more, but these I could find with a quick search):
- https://github.com/modxcms/revolution/issues/5566
- https://github.com/modxcms/revolution/issues/11120

with the proposed changes it works like this:
![modx templatetvs dd](https://cloud.githubusercontent.com/assets/445501/3356754/2e7dfc9c-fac6-11e3-8e34-87775556054f.gif)