[metacity] libmetacity: add appmenu class to button

[Alberts Muktupāvels <muktupavels src gnome org>]

commit fa4d02c1125ee7ad16449e57972edbf7a2d9d29e
Author: Alberts Muktupāvels <alberts muktupavels gmail com>
Date:   Thu Nov 22 13:26:51 2018 +0200

    libmetacity: add appmenu class to button

 libmetacity/meta-theme-gtk.c | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/libmetacity/meta-theme-gtk.c b/libmetacity/meta-theme-gtk.c
index a7123b00..33991cc8 100644
--- a/libmetacity/meta-theme-gtk.c
+++ b/libmetacity/meta-theme-gtk.c
@@ -654,6 +654,8 @@ get_class_from_button_type (MetaButtonType type)
     return "maximize";
   else if (type == META_BUTTON_TYPE_MINIMIZE)
     return "minimize";
+  else if (type == META_BUTTON_TYPE_APPMENU)
+    return "appmenu";
 
   return NULL;
 }