Draft: gesturestylus: honor `GtkGestureSingle:button` property

Use the GtkGestureSingle:button property to determine which buttons count for stylus contact. Press/release events for other buttons will not be consumed by the gesture.

With the property's default value of 0, any button press will count as contact, which is approximately the historical behavior, to ensure some measure of backwards compatibility.

Most users will want to set the property to 1, so that only button 1 registers as contact.