Rework custom agenda views

The agendas for monthly reviews and notes are gone, I don't use them.  My
monthly review does not look at the agenda at all, only at my backlog.
Searching notes is done using Org's search functionality, and it's working very
fine.

The „Everything“ agenda has been renamed to „Unsupervised“.  The name
„Everything“ is not apt anymore, as this view clusters items that are not
displayed in the main agenda or are stuck.  The name „Unsupervised“ is a little
bit better, but may also change in the future.

Add a new view for listing all items that are not periodic and not part of the
personal backlog (i.e., not tagged with SOMEWHEN).  This list should help
getting an overview over the current workload of real TODO tasks.  Note that
both periodic tasks (tagged with PERIODIC, i.e., series of tasks) and regular
tasks (tagged with REGULAR, i.e., repeating tasks) are excluded from this list.

Descriptions of agenda views have been extended whereever it felt necessary.

init.el

@@ -986,7 +986,7 @@ With given ARG, display files in `db/important-document-path’."
                 (search . "%-8c "))
 
               org-agenda-custom-commands
-              `(("A" "Main List"
+              `(("A" "Main Agenda"
                  ((agenda
                    ""
                    ((org-agenda-entry-types '(:timestamp :sexp :scheduled :deadline))
@@ -998,8 +998,15 @@ With given ARG, display files in `db/important-document-path’."
                     (org-deadline-warning-days 30)))
                   (tags "REFILE"
                         ((org-agenda-files (list db/org-default-refile-file))
-                         (org-agenda-overriding-header "Things to refile")))))
+                         (org-agenda-overriding-header "Things to refile (make it empty!)")))))
-                ("E" "Everything"
+
+                ("O" "Open, non-periodic TODOs"
+                 ((tags-todo "-PERIODIC-SOMEWHEN-REGULAR"
+                             ((org-agenda-overriding-header "List of open, non-periodic TODO items")
+                              (org-use-tag-inheritance t)
+                              (org-agenda-sorting-strategy '(deadline-down priority-down))))))
+
+                ("U" "Unsupervised (Waiting, Unscheduled, Stuck)"
                  ((tags-todo "/WAIT"
                              ((org-agenda-overriding-header "Tasks requiring response/input")))
                   (tags-todo "-HOLD-READ-SOMEWHEN/-DONE"
@@ -1010,34 +1017,22 @@ With given ARG, display files in `db/important-document-path’."
                                '(priority-down time-up category-keep))))
                   (stuck ""
                          ((org-agenda-overriding-header "Stuck Tasks")))))
-                ("S" "Somewhen"
+
+                ("S" "Somewhen (Do if nothing else to do, i.e., personal backlog)"
                  ((tags-todo "SOMEWHEN/-CANC-DONE|READ/-CANC-DONE"
                              ((org-agenda-overriding-header "Things to do or read somewhen")
                               (org-agenda-todo-ignore-with-date t)
                               (org-tags-match-list-sublevels nil)))
                   (tags-todo "/HOLD"
                              ((org-agenda-overriding-header "Tasks on Hold")))))
+
                 ("W" "Weekly Review"
                  ((agenda ""
                           ((org-agenda-span 7)
                            (org-agenda-archives-mode t)
                            (org-agenda-dim-blocked-tasks nil)
-                           (org-agenda-skip-deadline-prewarning-if-scheduled t)))))
+                           (org-agenda-skip-deadline-prewarning-if-scheduled t)))))))
-                ("M" "Monthly Preview"
+
-                 ((db/org-agenda-list-deadlines
-                   ""
-                   ((org-agenda-overriding-header "Deadlines")
-                    (org-agenda-sorting-strategy '(deadline-up priority-down))
-                    (org-deadline-warning-days 90)))
-                  (agenda ""
-                          ((org-agenda-span 'month)
-                           (org-agenda-dim-blocked-tasks nil)
-                           (org-deadline-warning-days 0) ; covered by display above
-                           ))))
-                ("N" "Notes" tags "NOTE"
-                 ((org-agenda-overriding-header "Notes")
-                  (org-use-tag-inheritance nil)
-                  (org-agenda-prefix-format '((tags . "  ")))))))
   :config (progn
             ;; avoid important buffers to end up in `org-agenda-new-buffers’ by
             ;; opening them manually