Because there are multiple different types of styluses that can be used with
tablets, we have to have some sort of identifier for them attached to the
GdkDeviceTool, especially since knowing the actual tool type for a GdkDeviceTool
is necessary for matching up a GdkDeviceTool with it's appropriate
GdkInputSource in Wayland (eg. matching up a GdkDeviceTool eraser with the
GDK_SOURCE_ERASER GdkInputSource of a wayland tablet).
Signed-off-by: Stephen Chandler Paul <thatslyude@gmail.com>