keditbookmarks lives in applications, making it very likely that it is
not installed. this does already raise an error window explaining that
the binary is not installed, unfortunately that is fairly poor user
experience. instead do not show the edit action if we have no editor
installed.
this is a bit unfortunate design-wise KBookmarkOwner is meant to control
if the edit action is shown, it does however encourage deriving from it
to control this behavior making it more than likely that devs simply derive
and override enableOption always returning true to enable all features
(while technically unnecessary). to deal with this we make the executable
lookup in the menu implementation, in addition to checking enableOption.
effectively we now have "does the owner want us to show the edit option,
and if so can we even edit?"