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.

README.org

@@ -481,6 +481,7 @@ Expands into a call to ~org-ql-select~ with the same arguments.  For convenience
 +  Variable =org-ql-block-header=, which overrides the default header in =org-ql-block= agenda blocks.
 +  Predicate =(path)=.
 +  Option =org-ql-views= may now be customized in a guided, structured way with the customization UI (e.g. =M-x customize-option RET org-ql-views RET=, or press =c= in the =org-ql-view-sidebar= buffer).
++  Enable more Org Agenda commands in =org-ql-view= buffers (e.g. setting deadlines and scheduling).  (Fixes [[https://github.com/alphapapa/org-ql/issues/35][#35]].  Thanks to [[https://github.com/mz-pdm][Milan Zamazal]] and [[https://github.com/mskorzhinskiy][Mikhail Skorzhinskii]].)
 
 *Changed*
 +  Predicate =heading= now accepts multiple regexps, which are matched with boolean =AND=.

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)))))