Bug 1042116 - Add bookmarks provider r=kgrandon

Hi folks,

This PR is for supporting the bookmarks store, took a bit more of time, since I decided to reuse the Places one, since it's basically the same content.

In this PR you can find a generalisation (actually same code) of the Places provider into a new provider called SyncProvider, with Places and Bookmarks using this generalisation.

Thanks!
This is removed in the latest master, and we simply enable all providers now. Can you update it to follow what we do in master? Sorry about that.