Bookmark random IDs could collide

When creating a new bookmark (which currently only happens in ephy_add_bookmark_popover_show(), but will also occur during bookmark import since !546 (merged)) we use ephy_sync_utils_get_random_sync_id() to generate a random ID for the bookmark. Although extremely unlikely, it's possible this ID could collide with an existing one, in which case trouble would result. We should check to make sure no existing bookmarks have the same ID before passing it to ephy_bookmark_new().

We need to do a similar check when importing existing bookmarks from GVDB. If the imported bookmark has the same ID and URL as an existing bookmark, we can say they're duplicates, but otherwise we should generate a new random ID for the bookmark being imported.