WIP: Filters

From @agpchil 's code, I did my take on the filtering. Not very clear which is the best approach. As it is now.


We don't have a way to deactivate a docset via elisp. The 'active' docsets are all the docsets installed in the machine. Then the filtering is ment to be done on a per-buffer basis.

If there's no modification of the per-buffer, we use the global one, which has everything.