Add a way to ignore devices for tablet mode

The tablet mode code checks whether we are currently in tablet mode
based on whether or not we have a touch screen and a pointer. For some
devices, a touch screen or pointer device may be detected but we would
like to ignore it, since it incorrectly influences the tablet mode.

This reads the udev tag "kwin_ignore_tabletmode" from libinput devices
and will skip devices with that tag when trying to determine whether we
are in tablet mode or not.