Outliner: better support for paintcurves ('Blender File' view)

- paintcurves were hitting an assert in outliner_add_element()
- missing outliner update when adding a paintcurve
- paintcurves were not showing an icon [they dont have a dedicated icon,
took the one used elsewhere]

Reviewers: brecht

Differential Revision: https://developer.blender.org/D4445

source/blender/windowmanager/WM_types.h

@@ -246,7 +246,7 @@ typedef struct wmNotifier {
 #define	NC_GEOM				(16<<24)
 #define NC_NODE				(17<<24)
 #define NC_ID				(18<<24)
-/* (19<<24) is free */
+#define NC_PAINTCURVE		(19<<24)
 #define NC_MOVIECLIP			(20<<24)
 #define NC_MASK				(21<<24)
 #define NC_GPENCIL			(22<<24)