status_hilite for Xp and Exp by percent rules

status_hilite for Xp and Exp by percent rules

Extend support for highlight rules that specify percentages from HP
and spell power to experience level and experience points.  For both
of those, the percentage is based on progress from the start of the
current Xp level to the start of the next Xp level.  100% isn't
possible so is used to enable highlighting a special case:  1 point
shy of next level, most likely to occur after losing a level.

This is something I had in mind a long time ago and then forgot all
about until fiddling with the final disclosure of experience points
recently.  It turned out to be trickier than expected because it needs
to check whether Xp should have a status update when it hasn't changed
but Exp has gone up.  The latter might hit a percentage threshold that
switches to another highlight rule.  Fortunately changes to Exp, at
least that aren't part of level gain or loss (which always trigger
status updating), are all funnelled through a single place (I hope).