The proxy API is the one that we would like to expose for applications
and the other API is used internally when implementing modules or
factories.
The current pw_core object is really a context for all objects so
name it that way. It also makes it possible to rename pw_core_proxy
to pw_proxy later.
Make a set-prop command to set a property from the config file
into a pw_properties. Pass this to the pw_core_new() and the
main-loop to tweak some stuff.
Move some warns to errors
