Assistant: Make "Bookmarks Menu" the default bookmarks folder

Assistant doesn't show the bookmarks toolbar by default so it's more
reasonable to preselect the "Bookmarks Menu" folder when a user adds
a bookmark.

Change-Id: If1b92c1decb280e96e5c58a56d6948eec6e44813
Reviewed-by: Karsten Heimrich <karsten.heimrich@digia.com>
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>