UI: Show library status icons in search menus, rather than letter prefix

Rather than a `L` (linked), `O` (overridden) or `M` (missing) prefix for the
name, show the existing library status icons. See D7999 for screenshots.

Note that when using preview icons, or if the search menu contains items with
own icons (e.g. brush icons), we still fallback to the prefix solution.

Zero or fake user is still indicated with a prefix.

Differential Revision: https://developer.blender.org/D7999

Reviewed by: Bastien Montagne, William Reynish

source/blender/editors/include/UI_interface_icons.h

@@ -106,6 +106,7 @@ struct PreviewImage *UI_icon_to_preview(int icon_id);
 
 int UI_rnaptr_icon_get(struct bContext *C, struct PointerRNA *ptr, int rnaicon, const bool big);
 int UI_idcode_icon_get(const int idcode);
+int UI_library_icon_get(const struct ID *id);
 
 #ifdef __cplusplus
 }