log: replace "priority" by "level" everywhere

log: replace "priority" by "level" everywhere

Now that we don't use syslog, "level" does sound more commonplace. We
should change it while there is still nobody using it.
Also leave some space between the integers of the xkb_log_level enum
values, if we ever need to shove more in between.

Signed-off-by: Ran Benita <ran234@gmail.com>