Add ParamModeBase and ParamMode, change all parameter modes to inherit from ParamMode

- Type of the extension used to store data is a template parameter
- The extension is automatically unset when the mode is unset
- Handlers inheriting from ParamMode have to provide OnSet() and SerializeParam(); may optionally provide OnUnset()
- Transparently handle the case when OnSet() modifies the mode parameter
- Remove Channel::custom_mode_params map; ask the mode handlers to serialize their parameters instead