Improving the bookmark capabilities of okular.
Adding a BookmarkManager class that takes care of handling them in a proper way.

Adding a new tab in the sidebar to manage them:
- shows all the bookmarks in all the documents, or just in the current document
- cliking on a bookmark will bring to that bookmark, loading the document if necessary
- editing a bookmark title is possible either using F2 or right click -> Rename
- right click -> Remove to remove a bookmark

The only way to add new bookmark for the moment is the "old" way, ie right click -> Add bookmark, and it's not possible add more than one bookmark per page (but the system does not limit that).

The whole system should not be too difficult to understand, I will check with our usability master Florian next days.