Give property classes names so you can distinguish between
property classes with same interface but different purpose. [ok]

Implement iPcMovable and iPcMovableConstraint. An iPcMovable
will be able to have pointers to several constraints.
Implement iPcMeshSelect so it moves the mesh through
the iPcMovable if present (if not present use iPcMesh
directly). Note that a constraint can allow a particular
move operation, disallow a move, or partly allow a
move (for example is an obstacle if half-way in
the desired movement vector). [ok]

Implement iPcInventoryConstraints for behaviour/game-defined
constraints in inventory system. [ok]

Allow SendMessage() to return a result from behaviour
to property class (i.e. in order to implement constraints
for example). 

Implement iPcSolid for collision detection. One of the
iPcMovableConstraint implementations can use this to
constrain movement according to collision detection rules. [ok]

iPcSolid can also be used by a iPcForce property class
which will implement a force based physics system. [ok]

Add the ability to all property classes to explicitely
set target/companion property classes (i.e. set the mesh to
use together with mesh select and so on).

Think about the issue with property classes refering
to other property classes that might get deleted...
How do we clean that up? iPcMovable currently keeps
a reference incremented.

Create an introspection like interface with Get/Set
attributes for all property classes to making scripting
easier.

Add more optional messages to the behaviour from property
classes so that more control is possible.

Maybe add SetEntity() to iCelBehaviour so that SetBehaviour
can set it automatic? [ok]

Let pcsolid support more than one collider? Probably better.

Implement movement through portals in pcmovable.

IMPORTANT: unique names for entities! Can we assume current
name of entities are unique? Unique name generator. solved with IDs [ok]
IMPORTANT: Consider class names for entities in addition to
names? i.e. name: "small box", class: "box". [we'll not do that]

Add support for KEY/VALUE system for placement of entities.

Use reporter to report error conditions (especially in persistence
layer). [ok]

Save sector position in pccamera. [ok]

pcinput_... to pckeyinput_... messages... [ok]

fix persistence layer when loading entities or property classes that are
referenced multiple times.