[MonoDevelop] Migration to Mono.Addins

[Lluis Sanchez]

Hi,

In the past weeks I've been working on replacing the MD add-in engine by
Mono.Addins. Now the work is ready to be committed, and I plan to do it
tomorrow. This migration does not require big changes in the code since
the add-in model is very similar, although many files will be modified
due to changes in namespace, class and method names. The .addin file is
also a bit different since extension points are declared in a different
way.

I've been testing the updated MD in the past days and it seems to work
fine. If you find any problem after the migration please let me know.

Thanks!
Lluis.