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/support/dbus.h

@@ -31,6 +31,11 @@ extern "C" {
 
 #include <spa/support/loop.h>
 
+/**
+ * \addtogroup spa_support
+ * \{
+ */
+
 #define SPA_TYPE_INTERFACE_DBus		SPA_TYPE_INFO_INTERFACE_BASE "DBus"
 
 #define SPA_VERSION_DBUS		0
@@ -133,6 +138,10 @@ spa_dbus_get_connection(struct spa_dbus *dbus, enum spa_dbus_type type)
 	return res;
 }
 
+/**
+ * \}
+ */
+
 #ifdef __cplusplus
 }  /* extern "C" */
 #endif