#340 Bookmarks - If bookmark doesn't exist, it should be disabled

[Reijo Tomperi]

This is a simple usability feature. If bookmark doesn't
exist, you shouldn't be able to select it from the
list. It should be for example set as disabled.

This is why:
- If bookmark is disabled, newbie knows that he/she
can't use that bookmark for some reason. Most likely
the person realizes that there are no bookmarks set yet
and he/she finds a way to set it.
- Advanced users can check from the list with some
names disabled what bookmarks are not used and what are
used.
- If there is will be no action behind a button, it
should be disabled or removed by general usability
guidelines. At any case, it should not look like a
normal working button.