org-agenda.el: Allow to move backward/forward blocks

* org-agenda.el (org-agenda-find-same-or-today-or-agenda): Use
`org-agenda-backward-block'.
(org-agenda-backward-block): Renamed and rewritten from
`org-agenda-goto-block-beginning'.
(org-agenda-forward-block): New command.
(org-agenda-mode-map): Remap `backward-paragraph' and
`forward-paragraph' to `org-agenda-backward-block' and
`org-agenda-forward-block' respectively.

Thanks to Bart Bunting for raising this.

lisp/org-agenda.el

@@ -2318,6 +2318,10 @@ The following commands are available:
 
 (org-defkey org-agenda-mode-map [mouse-2] 'org-agenda-goto-mouse)
 (org-defkey org-agenda-mode-map [mouse-3] 'org-agenda-show-mouse)
+
+(define-key org-agenda-mode-map [remap forward-paragraph] 'org-agenda-forward-block)
+(define-key org-agenda-mode-map [remap backward-paragraph] 'org-agenda-backward-block)
+
 (when org-agenda-mouse-1-follows-link
   (org-defkey org-agenda-mode-map [follow-link] 'mouse-face))
 (easy-menu-define org-agenda-menu org-agenda-mode-map "Agenda menu"
@@ -7808,27 +7812,40 @@ Negative selection means regexp must not match for selection of an entry."
        (text-property-any (point-min) (point-max) 'org-today t)
        (text-property-any (point-min) (point-max) 'org-agenda-type 'agenda)
        (and (get-text-property (min (1- (point-max)) (point)) 'org-series)
-	    (org-agenda-goto-block-beginning))
+	    (org-agenda-backward-block))
        (point-min))))
 
-(defun org-agenda-goto-block-beginning ()
-  "Go the agenda block beginning."
+(defun org-agenda-backward-block ()
+  "Move backward by one agenda block."
   (interactive)
-  (if (not (derived-mode-p 'org-agenda-mode))
-      (error "Cannot execute this command outside of org-agenda-mode buffers")
-    (let (dest)
-      (save-excursion
-	(unless (looking-at "\\'")
-	  (forward-char))
-	(let* ((prop 'org-agenda-structural-header)
-	       (p (previous-single-property-change (point) prop))
-	       (n (next-single-property-change (or (and (looking-at "\\`") 1)
-						   (1- (point))) prop)))
-	  (setq dest (cond ((eq n (point-at-eol)) (1- n)) (p (1- p))))))
-      (if (not dest)
-	  (error "Cannot find the beginning of the blog")
-	(goto-char dest)
-	(move-beginning-of-line 1)))))
+  (org-agenda-forward-block 'backward))
+
+(defun org-agenda-forward-block (&optional backward)
+  "Move forward by one agenda block.
+When optional argument BACKWARD is set, go backward"
+  (interactive)
+  (cond ((not (derived-mode-p 'org-agenda-mode))
+	 (user-error
+	  "Cannot execute this command outside of org-agenda-mode buffers"))
+	((looking-at (if backward "\\`" "\\'"))
+	 (message "Already at the %s block" (if backward "first" "last")))
+	(t (let ((pos (prog1 (point)
+			(ignore-errors (if backward (backward-char 1)
+					 (move-end-of-line 1)))))
+		 (f (if backward
+			'previous-single-property-change
+		      'next-single-property-change))
+		 moved dest)
+	     (while (and (setq dest (funcall
+				     f (point) 'org-agenda-structural-header))
+			 (not (get-text-property
+			       (point) 'org-agenda-structural-header)))
+	       (setq moved t)
+	       (goto-char dest))
+	     (if moved (move-beginning-of-line 1)
+	       (goto-char (if backward (point-min) (point-max)))
+	       (move-beginning-of-line 1)
+	       (message "No %s block" (if backward "previous" "further")))))))
 
 (defun org-agenda-later (arg)
   "Go forward in time by the current span.