backends: Add support for Wacom stylus tertiary-button-action

The tertiary-button-action (see bug 790028) is a place for g-c-c to store
the action which should be performed when a stylus' third button is pressed.
Pressing this button is signaled as a BTN_STYLUS3 event from the kernel or
X11 button 8.

https://bugzilla.gnome.org/show_bug.cgi?id=790033

src/backends/meta-input-settings-private.h

@@ -111,7 +111,8 @@ struct _MetaInputSettingsClass
                                   ClutterInputDevice         *device,
                                   ClutterInputDeviceTool     *tool,
                                   GDesktopStylusButtonAction  primary,
-                                  GDesktopStylusButtonAction  secondary);
+                                  GDesktopStylusButtonAction  secondary,
+                                  GDesktopStylusButtonAction  tertiary);
   gboolean (* has_two_finger_scroll) (MetaInputSettings  *settings,
                                       ClutterInputDevice *device);
 };