[mutter/gnome-3-28] theme: add ".appmenu" class to the appmenu button

[Marco Trevisan <marcotrevi src gnome org>]

commit a98eb107be5e2611ff85f61ade0bff5d485ddce5
Author: Marco Trevisan (Treviño) <mail 3v1n0 net>
Date:   Wed Mar 28 23:46:54 2018 -0500

    theme: add ".appmenu" class to the appmenu button
    
    So it does gtk headerbar, so mutter should do.
    
    
    (cherry picked from commit 96141e28f9c5721e30c89eec34650c9f676fb608)

 src/ui/theme.c | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/src/ui/theme.c b/src/ui/theme.c
index 2b3cd5eaf..8786b17f3 100644
--- a/src/ui/theme.c
+++ b/src/ui/theme.c
@@ -712,6 +712,8 @@ get_class_from_button_type (MetaButtonType type)
       return "maximize";
     case META_BUTTON_TYPE_MINIMIZE:
       return "minimize";
+    case META_BUTTON_TYPE_APPMENU:
+      return "appmenu";
     default:
       return NULL;
     }