Add: (org-ql-view--format-element) Set org-agenda-type to search

Some org-agenda commands check the org-agenda-type text property to
ensure that the command is valid to run in that type of agenda.
Notably this fixes running commands org-agenda-schedule and
org-agenda-deadline.

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

Fixes #35.

org-ql-view.el

@@ -490,6 +490,7 @@ return an empty string."
            ;; FIXME: Use proper prefix
            (concat "  " it)
            (org-add-props it properties
+             'org-agenda-type 'search
              'todo-state todo-keyword
              'tags tag-list
              'org-habit-p habit-property)))))