It is nice to have finer granularity for device types to allow for
configurations such as using `naturalScroll` on touchpads, but not on
regular pointer devices such as mice.
Previously, if rc.xml defined only non-default libinput categories,
no default category was created. This meant that configure_libinput()
might totally skip configuring some devices even with default
settings, like tap-to-click.
Fix this by making sure that a default category is always created.
