Instead of everyone overriding save_to() and set_property() and doing
a pretty asymmetric job of implementing the various properties, let's
add a bit of structure here.
Object properties are now represented by a Core::Property. Properties
are registered with a getter and setter (optional) in constructors.
I've added some convenience macros for creating and registering
properties, but this does still feel a bit bulky. We'll have to
iterate on this and see where it goes.
Add a SetInspectedObject call that tells us which Core::Object a remote
client is currently looking it. Objects get notified when they gain
their first inspector, and when they lose their last one.
This was only used by HashTable::dump() which I used when doing the
first HashTable implementation. Removing this allows us to also remove
most includes of <AK/kstdio.h>.
