Arcane Shield fix

Allow damage shields to merge

-This change was originally intended to fix a bug with Arcane Shield
(http://forums.te4.org/viewtopic.php?f=42&t=44895), but I ended up
creating an on_merge function for timed effect DAMAGE_SHIELD so now
anything that creates damage shields will intelligently overwrite an
existing shield.

-Will replace an existing damage shield entirely if both the shield
value and duration would be greater on the new shield.

-Will update the current shield value (but not max value) of an
existing shield if the new value would be greater. We keep the old
max value because the player may want to buff their shield with Aegis.

-If we updated an existing shield, we will refresh the current duration
based on the new shield's duration unless the duration has already been
refreshed 20 times. This checks the same property used by Weapon of
Light and Bathe in Light, which is why I allowed refreshing 20 times
instead of a smaller amount.

-Will never update or replace an existing shield if the
current duration is greater than the new shield's duration. We
can't know whether the player values shield value or duration more, and
updating the shield value of an existing damage shield with greater
duration would introduce power creep.

-Will never merge a reflection shield with a normal shield. For merging
two reflection shields, the lesser reflect value is used.

-Many shield sources (e.g., Shielding rune) are still disabled while
the player has a damage shield. This is fine because the player can
always cancel the current effect if they want to replace it. For
merging, we're mainly concerned about back-end sources like Arcane
Shield.

-EFF_DAMAGE_SHIELD and EFF_PSI_DAMAGE_SHIELD replace each other upon
activation (existing behavior). I may address that in a different
branch, but I need to investigate the consequences of changing it.