Fix alphapapa#35: set org-agenda-type for org-ql buffer content

Some functions in org-agenda checking against org-agenda-type variable to
ensure that this action is valid to run in that type of agenda. Notably
this fix running 'org-agenda-schedule' and 'org-agenda-deadline'
functions.

Also setting agenda type to 'search' prohibits some of the functions that
are still not working on in these buffers, like
"org-agenda-do-date-earlier".

org-ql-agenda.el

@@ -574,6 +574,7 @@ return an empty string."
            (org-add-props it properties
              'todo-state todo-keyword
              'tags tag-list
+             'org-agenda-type 'search
              'org-habit-p habit-property)))))
 
 (defun org-ql-agenda--add-faces (element)