UI: add optional tip callback to uiBut, and use it for per-item tooltips in UIList.

When defined, uiBut->tip_func is called when button's tip is generated. This allows
for advanced, dynamic generation of tooltips.

For now, only used by UIList, which can now optionaly use a given string property
of each item for its tooltip.

Thanks to Campbell for the reviews!

source/blender/editors/interface/interface_intern.h

@@ -254,7 +254,11 @@ struct uiBut {
 	uiLink *link;
 	short linkto[2];  /* region relative coords */
 	
-	const char *tip, *lockstr;
+	const char *tip;
+	uiButToolTipFunc tip_func;
+	void *tip_argN;
+
+	const char *lockstr;
 
 	BIFIconID icon;
 	bool lock;