clutter/evdev: Add support for BTN_STYLUS3

BTN_STYLUS3 is defined by the Linux 4.15 kernel and is sent when the
third button on a stylus is pressed. At the moment, only Wacom's "Pro
Pen 3D" has three stylus buttons. Pressing this button triggers a button
8 event to be sent under X11, so we use the same mapping here.

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