Add read-later bookmarks

[raucao]

Adds support for folders to the RS bookmarks module as well as Webmarks. This means apps/clients can now choose to sync only the readlater bookmarks for example (or other folders like maybe browser bookmarks or user-defined ones).

The first new folder is for URLs that are saved for later. Bookmarks can be saved there by checking "save for later" when adding the bookmark. Naturally, they can then be archived from there, i.e. moved to the archive folder (the default for all bookmarks at the moment).