Allow packages to change the hl-line overlay priority

* lisp/hl-line.el (hl-line-overlay-priority): New variable
(bug#11509).
(hl-line-make-overlay): Use it.

etc/NEWS

@@ -1991,6 +1991,11 @@ Shift while typing 'C-a', i.e. 'C-S-a', will now highlight the text.
 
 ** Miscellaneous
 
+---
+*** New variable 'hl-line-overlay-priority'.
+This can be used to change the priority of the hl-line overlays.
+
++++
 *** New command 'mailcap-view-file'.
 This command will open a viewer based on the file type, as determined
 by ~/.mailcap and related files and variables.

lisp/hl-line.el

@@ -125,6 +125,9 @@ This variable is expected to be made buffer-local by modes.")
 (defvar hl-line-overlay-buffer nil
   "Most recently visited buffer in which Hl-Line mode is enabled.")
 
+(defvar hl-line-overlay-priority -50
+  "Priority used on the overlay used by hl-line.")
+
 ;;;###autoload
 (define-minor-mode hl-line-mode
   "Toggle highlighting of the current line (Hl-Line mode).
@@ -152,7 +155,7 @@ line about point in the selected window only."
 
 (defun hl-line-make-overlay ()
   (let ((ol (make-overlay (point) (point))))
-    (overlay-put ol 'priority -50)           ;(bug#16192)
+    (overlay-put ol 'priority hl-line-overlay-priority) ;(bug#16192)
     (overlay-put ol 'face hl-line-face)
     ol))