combobox: add a "combobox-entry" style class for comboboxes with entries

This way themes can use GtkComboBox.combobox-entry to match that
specific case, and GtkComboBox.combobox-entry .button to match the
button/arrow side of the widget.

gtk/gtkstylecontext.h

@@ -170,6 +170,15 @@ struct _GtkStyleContextClass
  */
 #define GTK_STYLE_CLASS_ENTRY "entry"
 
+/**
+ * GTK_STYLE_CLASS_COMBOBOX_ENTRY:
+ *
+ * A CSS class to match combobox entries.
+ *
+ * This is used by #GtkComboBox.
+ */
+#define GTK_STYLE_CLASS_COMBOBOX_ENTRY "combobox-entry"
+
 /**
  * GTK_STYLE_CLASS_BUTTON:
  *