Implement settings interface for plugins

Add a fairly easy to use settings interface that can be passed to
plugins, and replaced the old character string system that was being
used before.  The new data interface allows for an easier method of
getting/altering settings for plugins, and is built to be serializable
to/from JSON.

Also, removed another wxFormBuilder file that was no longer in use.

libobs/obs-scene.c

@@ -34,7 +34,7 @@ static const char *scene_getname(const char *locale)
 	return "Scene";
 }
 
-static void *scene_create(const char *settings, struct obs_source *source)
+static void *scene_create(obs_data_t settings, struct obs_source *source)
 {
 	struct obs_scene *scene = bmalloc(sizeof(struct obs_scene));
 	scene->source     = source;