bookmarks: expose manager and updated status

For plugins to interact with bookmarks they need access to the bookmark
manager and whether an item has been updated.
BookmarkItem: rename status methods; only allow clear

There doesn't seem to be a case for bookmarks being externally marked as
updated, so plugins need only be able to clear the status.

These names are more concise and can read as a sentence like "if
bookmark has updated."
Merge branch 'plugin-bookmark-support' into next