Implement next-error and previous-error functionality for sparse trees

* lisp/org.el (org-occur-next-match): New function.
(org-mode): Set the variable `next-error-function'.
(org-highlight-new-match): Add an `org-type' property to the overlays.
* doc/org.texi (Sparse trees): Document the next-error / previous-error
functionality.

After a sparse tree construction, `M-g n' and `M-g p' will now jump to
the location of matches.

doc/org.texi

@@ -1429,8 +1429,13 @@ editing command@footnote{This depends on the option
 @code{org-remove-highlights-with-change}}, or by pressing @kbd{C-c C-c}.
 When called with a @kbd{C-u} prefix argument, previous highlights are kept,
 so several calls to this command can be stacked.
+@orgcmdkkc{M-g n,M-g M-n,next-error}
+Jump to the next sparse tree match in this buffer.
+@orgcmdkkc{M-g p,M-g M-p,previous-error}
+Jump to the previous sparse tree match in this buffer.
 @end table
 
+
 @noindent
 @vindex org-agenda-custom-commands
 For frequently used sparse trees of specific search strings, you can

lisp/org.el

@@ -4720,6 +4720,8 @@ The following commands are available:
   ;; Beginning/end of defun
   (org-set-local 'beginning-of-defun-function 'org-beginning-of-defun)
   (org-set-local 'end-of-defun-function 'org-end-of-defun)
+  ;; Next error for sparse trees
+  (org-set-local 'next-error-function 'org-occur-next-match)
   ;; Make sure dependence stuff works reliably, even for users who set it
   ;; too late :-(
   (if org-enforce-todo-dependencies
@@ -11981,6 +11983,32 @@ that the match should indeed be shown."
 	(message "%d match(es) for regexp %s" cnt regexp))
     cnt))
 
+(defun org-occur-next-match (&optional n reset)
+  "Function for `next-error-function' to find sparse tree matches.
+N is the number of matches to move, when negative move backwards.
+RESET is entirely ignored - this function always goes back to the
+starting point when no match is found."
+  (let* ((limit (if (< n 0) (point-min) (point-max)))
+	 (search-func (if (< n 0)
+			  'previous-single-char-property-change
+			'next-single-char-property-change))
+	 (n (abs n))
+	 (pos (point))
+	 p1)
+    (catch 'exit
+      (while (setq p1 (funcall search-func (point) 'org-type))
+	(when (equal p1 limit)
+	  (goto-char pos)
+	  (error "No more matches"))
+	(when (equal (get-char-property p1 'org-type) 'org-occur)
+	  (setq n (1- n))
+	  (when (= n 0)
+	    (goto-char p1)
+	    (throw 'exit (point))))
+	(goto-char p1))
+      (goto-char p1)
+      (error "No more matches"))))
+
 (defun org-show-context (&optional key)
   "Make sure point and context are visible.
 How much context is shown depends upon the variables
@@ -12044,6 +12072,7 @@ entire tree."
   "Highlight from BEG to END and mark the highlight is an occur headline."
   (let ((ov (make-overlay beg end)))
     (overlay-put ov 'face 'secondary-selection)
+    (overlay-put ov 'org-type 'org-occur)
     (push ov org-occur-highlights)))
 
 (defun org-remove-occur-highlights (&optional beg end noremove)