doc: reorganize Doxygen groups/modules to make more sense

The Doxygen "Modules" page is not very illuminative, as different parts
of the API are mixed together and not all parts are included.

Try to address this:

Put all parts of the public API to some Doxygen group, usually one group
per header file. Use short, systematic names.

Make these groups sub-groups of a few top-level groups, roughly
corresponding to the different logical parts of the API (core, impl,
stream, filter, spa, utilities).

spa/include/spa/support/cpu.h

@@ -34,12 +34,12 @@ extern "C" {
 #include <spa/utils/defs.h>
 #include <spa/utils/hook.h>
 
-/** \defgroup spa_support SPA Support Features
+/** \defgroup spa_cpu CPU
  *
  */
 
 /**
- * \addtogroup spa_support
+ * \addtogroup spa_cpu
  * \{
  */
 

spa/include/spa/support/dbus.h

@@ -31,8 +31,12 @@ extern "C" {
 
 #include <spa/support/loop.h>
 
+/** \defgroup spa_dbus DBus
+ *
+ */
+
 /**
- * \addtogroup spa_support
+ * \addtogroup spa_dbus
  * \{
  */
 

spa/include/spa/support/i18n.h

@@ -32,8 +32,12 @@ extern "C" {
 #include <spa/utils/hook.h>
 #include <spa/utils/defs.h>
 
+/** \defgroup spa_i18n I18N
+ *
+ */
+
 /**
- * \addtogroup spa_support
+ * \addtogroup spa_i18n
  * \{
  */
 

spa/include/spa/support/log.h

@@ -35,7 +35,7 @@ extern "C" {
 #include <spa/utils/defs.h>
 #include <spa/utils/hook.h>
 
-/** \defgroup spa_log SPA Logging
+/** \defgroup spa_log Log
  */
 
 /**

spa/include/spa/support/loop.h

@@ -33,8 +33,12 @@ extern "C" {
 #include <spa/utils/hook.h>
 #include <spa/support/system.h>
 
+/** \defgroup spa_loop Loop
+ *
+ */
+
 /**
- * \addtogroup spa_support
+ * \addtogroup spa_loop
  * \{
  */
 

spa/include/spa/support/plugin-loader.h

@@ -32,8 +32,12 @@ extern "C" {
 #include <spa/utils/hook.h>
 #include <spa/utils/defs.h>
 
+/** \defgroup spa_plugin_loader Plugin Loader
+ *
+ */
+
 /**
- * \addtogroup spa_support
+ * \addtogroup spa_plugin_loader
  * \{
  */
 

spa/include/spa/support/plugin.h

@@ -33,7 +33,7 @@ extern "C" {
 #include <spa/utils/dict.h>
 
 /**
- * \defgroup spa_handle SPA Plugin Handles
+ * \defgroup spa_handle Plugin Handle
  */
 
 /**

spa/include/spa/support/system.h

@@ -37,8 +37,12 @@ struct itimerspec;
 #include <spa/utils/defs.h>
 #include <spa/utils/hook.h>
 
+/** \defgroup spa_system System
+ *
+ */
+
 /**
- * \addtogroup spa_support
+ * \addtogroup spa_system
  * \{
  */
 

spa/include/spa/support/thread.h

@@ -36,8 +36,12 @@ extern "C" {
 #include <spa/utils/hook.h>
 #include <spa/utils/dict.h>
 
+/** \defgroup spa_thread Thread
+ *
+ */
+
 /**
- * \addtogroup spa_support
+ * \addtogroup spa_thread
  * \{
  */