GdkDeviceTool is an opaque object that can be used to identify a given
tool (eg. pens on tablets) during the app/device lifetime. Tools are only
set on non-master devices, and are owned by these.
The accounting functions are made private, the only public call on
GdkDeviceTool so far is gdk_device_tool_get_serial(), useful to identify
the tool across runs.
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