Add a way to get default settings

- Implement a means of obtaining default settings for an
   input/output/encoder.  obs_source_defaults for example will return
   the default settings for a particular source type.

 - Because C++ doesn't have designated initializers, use functions in
   the WASAPI plugin to register the sources instead.

plugins/win-wasapi/plugin-main.cpp

@@ -2,12 +2,12 @@
 
 OBS_DECLARE_MODULE()
 
-extern struct obs_source_info wasapiInput;
-extern struct obs_source_info wasapiOutput;
+void RegisterWASAPIInput();
+void RegisterWASAPIOutput();
 
 bool obs_module_load(uint32_t libobs_ver)
 {
-	obs_register_source(&wasapiInput);
-	obs_register_source(&wasapiOutput);
+	RegisterWASAPIInput();
+	RegisterWASAPIOutput();
 	return true;
 }