* rulesets/EntityProperty.h, rulesets/EntityProperty.cpp:
Refactor so this property is always dynamic, and add an accessor.
* rulesets/Character.cpp, rulesets/Character.h: Remove the hard
coded entity references used for wield tool, and replace with
a dynamic property.
* rulesets/EntityProperty.cpp: Modify EntityProperty so it
shows its value as an empty ID if it is not set to anything,
and clears its reference if it is set to an empty string.
* rulesets/Character.cpp: Fix the code that handles an empty wield
argument, so it clears the wielded entity correctly and efficiently.
* rulesets/attributes.h, server/Persistor_impl.h,
server/Restorer.cpp: Remove right_hand_wield from the
persistence layer, pending its complete removal.
* rulesets/EntityProperty.h, rulesets/EntityProperty.cpp:
Make EntityProperty a more normal property by making it hold
a reference to its value, not the value itself.
* rulesets/Character.cpp, rulesets/Character.h: Use EntityProperty
for right_hand_wield attribute, so it stores the value long term
which I think is more efficient.
* tests/EntityPropertytest.cpp: Add a complete set of unit tests for
EntityProperty.
* rulesets/EntityProperty.cpp: Fix and instrument the code based
on the findings from the unit test.
