Improve debug output

[superdump]

Debugging what went wrong in OpenWebRTC is currently pretty difficult and requires quite a bit of knowledge of OpenWebRTC and/or GStreamer and/or WebRTC.

OpenWebRTC needs:

• Debug categories specific to the various objects and components involved in a WebRTC session
• Debug log output specific to all of the higher-level important stages of a WebRTC session life cycle

Just those two should enable identification of where in the code and when in the life cycle of the WebRTC session an error occurs which should speed debugging of operation significantly.

[superdump]

As a note: NiceAgent component-state-changed signal should be connected to and used.