fix(power): fix semantics of {max,set,refund}_XXX in SpellInfo()

[johnnylam88]

When set_<powerType> is given in SpellInfo(), use that as the
value to which the power should be set after the spell is
successfully cast. Ensure that the cost of the spell given by
<powerType> is met even if set_<powerType> is given.

If max_<powerType> is given in SpellInfo(), use that as the max
cost of the spell. Use getProperty() instead of directly
accessing the value so that any spell requirements are handled at
runtime. Note that power cost from set_<powerType> has
precedence over the cost from max_<powerType>.

Lastly, always evaluate refund_<powerType> to determine the
amount of power refunded after the spell is successfully cast.

With this change, a combo-point finisher spell can be defined in
two different ways:
SpellInfo(finisher combopoints=1 set_combopoints=0)
or
SpellInfo(finisher combopoints=1 max_combopoints=5)