input methods: Add a 'vertical text' hint

This has been requested in
https://bugzilla.gnome.org/show_bug.cgi?id=747150

gtk/gtkenums.h

@@ -1033,6 +1033,7 @@ typedef enum
  *     first word of each sentence
  * @GTK_INPUT_HINT_INHIBIT_OSK: Suggest to not show an onscreen keyboard
  *     (e.g for a calculator that already has all the keys).
+ * @GTK_INPUT_HINT_VERTICAL_WRITING: The text is vertical. Since 3.18
  *
  * Describes hints that might be taken into account by input methods
  * or applications. Note that input methods may already tailor their
@@ -1056,7 +1057,8 @@ typedef enum
   GTK_INPUT_HINT_UPPERCASE_CHARS     = 1 << 4,
   GTK_INPUT_HINT_UPPERCASE_WORDS     = 1 << 5,
   GTK_INPUT_HINT_UPPERCASE_SENTENCES = 1 << 6,
-  GTK_INPUT_HINT_INHIBIT_OSK         = 1 << 7
+  GTK_INPUT_HINT_INHIBIT_OSK         = 1 << 7,
+  GTK_INPUT_HINT_VERTICAL_WRITING    = 1 << 8
 } GtkInputHints;
 
 /**