Removed the findIntersectingObjects function from collision_util.

Because it relied on an object manager it was kind of rigid, etc. Soon it
will be replaced with a beautiful quadtree implementation for collision
resolution. Because this function is used extensively in the LocalServer::step
implementation, it has been commented out.
The ObjectManager now only provides a setter and a const getter for Objects.

This means that client code can no longer grab a reference and modify
that freely. Soon, a Quadtree will be cached inside the ObjectManager
which means all changes to Objects need to be known, etc. This system
does feel like it will end up slightly cleaner and explicit.
Added ObjectManager::ids that *just* returns the ids contained.
A new cache, which works without inheritance.

The client library has been disabled while the Label class is updated.
Added a copy constructor and assignment operator to the Cache.
Updated the Label cache implementation.

It didn't require any interface change except for the inheritance but it
was treated as private in client code so there wasn't an issue.
New expected release date: April 1st.