Add a csd style class

This will be used for client-side decorated windows.

https://bugzilla.gnome.org/show_bug.cgi?id=731187

docs/reference/gtk/gtk3-sections.txt

@@ -5966,6 +5966,7 @@ GTK_STYLE_CLASS_CALENDAR
 GTK_STYLE_CLASS_CELL
 GTK_STYLE_CLASS_COMBOBOX_ENTRY
 GTK_STYLE_CLASS_CHECK
+GTK_STYLE_CLASS_CSD
 GTK_STYLE_CLASS_CURSOR_HANDLE
 GTK_STYLE_CLASS_DEFAULT
 GTK_STYLE_CLASS_DESTRUCTIVE_ACTION

gtk/gtkstylecontext.h

@@ -248,6 +248,16 @@ struct _GtkStyleContextClass
  */
 #define GTK_STYLE_CLASS_RUBBERBAND "rubberband"
 
+/**
+ * GTK_STYLE_CLASS_CSD:
+ *
+ * A CSS class that gets added to windows which have
+ * client-side decorations.
+ *
+ * Since: 3.14
+ */
+#define GTK_STYLE_CLASS_CSD "csd"
+
 /**
  * GTK_STYLE_CLASS_TOOLTIP:
  *