Extension management gsoc2014

This week I discussed my GSoC work with Stefano and we agreed to merge my code in phases. I went through the first part, the extension management code,  in the last few days and ended up re-writing some parts of it. Most of the GUI code has been integrated into the Glade file, and a few functions that didn't turn out very useful in retrospect have been reworked. Overall, I think this second revision is much cleaner than the original. 

I haven't had a chance to test it very thorougly, but everything looks in order. I'm leaving on holidays in a few hours, but I should be able to look into any problems or questions should they arise.