* rulesets/ActivePropertyFactory_impl.h: Correct the header guard
comments.
* rulesets/SolidProperty.cpp, rulesets/SolidProperty.h: New property
to handler whether an entities bbox is solid or not.
* rulesets/EntityPropertyFactory.cpp, rulesets/EntityPropertyFactory.h,
rulesets/EntityPropertyFactory_impl.h: New property factory
for properties the contructor of which takes an entity pointer of
the owning entity.
* server/CorePropertyManager.cpp: Install the SOLID property on demand.
* data/basic.xml: Make area entities non-solid by default, so you don't
collide with them.
