proxy/core: refactor object creation
- core no longer exposes create_remote/local_object
- node, device & link have constructor methods to enable the create_remote_object functionality
- added WpImplNode to wrap pw_impl_node and allow creating "local" node instances
- added WpSpaDevice to wrap spa_device and allow creating "local" device instances
- exporting objects in all cases now happens by requesting FEATURE_BOUND from the proxy, eliminating the need for WpExported
- replaced WpMonitor by new, simpler code directly in module-monitor
- the proxy type lookup table in WpProxy is gone, we now use a field on the class structure of every WpProxy subclass and iterate through all the class structures instead; this is more flexible and extensible