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/plugin.h

@@ -32,6 +32,15 @@ extern "C" {
 #include <spa/utils/defs.h>
 #include <spa/utils/dict.h>
 
+/**
+ * \defgroup spa_handle SPA Plugin Handles
+ */
+
+/**
+ * \addtogroup spa_handle
+ * \{
+ */
+
 struct spa_handle {
 	/** Version of this struct */
 #define SPA_VERSION_HANDLE	0
@@ -208,6 +217,10 @@ int spa_handle_factory_enum(const struct spa_handle_factory **factory, uint32_t
 								  *  the filename of the plugin without the
 								  *  path or the plugin extension. */
 
+/**
+ * \}
+ */
+
 #ifdef __cplusplus
 }  /* extern "C" */
 #endif