spa: group the spa headers into doxygen modules

Subdirectories buffer, control, debug, monitor, pod, support and utils, others
are still missing. Headers are grouped either per subdirectory (e.g. buffer/
gets added to group spa_buffer) or per-file (e.g. spa_json is a separate
group), whatever seemed like the most sensible approach.

spa/include/spa/node/keys.h

@@ -29,6 +29,11 @@
 extern "C" {
 #endif
 
+/**
+ * \addtogroup spa_node
+ * \{
+ */
+
 /** node keys */
 #define SPA_KEY_NODE_NAME		"node.name"		/**< a node name */
 #define SPA_KEY_NODE_LATENCY		"node.latency"		/**< the requested node latency */
@@ -48,6 +53,10 @@ extern "C" {
 #define SPA_KEY_PORT_MONITOR		"port.monitor"		/**< this port is a monitor port */
 
 
+/**
+ * \}
+ */
+
 #ifdef __cplusplus
 }  /* extern "C" */
 #endif