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/utils/ansi.h

@@ -29,11 +29,13 @@
 extern "C" {
 #endif
 
-/** \defgroup spa_utils SPA Utils
+/**
+ * \defgroup spa_ansi ANSI codes
+ *
  */
 
 /**
- * \addtogroup spa_utils
+ * \addtogroup spa_ansi
  * \{
  */
 

spa/include/spa/utils/defs.h

@@ -37,11 +37,13 @@ extern "C" {
 #include <stddef.h>
 #include <stdio.h>
 
-/** \defgroup spa_utils SPA Utils
+/**
+ * \defgroup spa_utils_defs Miscellaneous
+ *
  */
 
 /**
- * \addtogroup spa_utils
+ * \addtogroup spa_utils_defs
  * \{
  */
 

spa/include/spa/utils/dict.h

@@ -34,7 +34,12 @@ extern "C" {
 #include <spa/utils/defs.h>
 
 /**
- * \addtogroup spa_utils
+ * \defgroup spa_dict Dictionary
+ *
+ */
+
+/**
+ * \addtogroup spa_dict
  * \{
  */
 

spa/include/spa/utils/hook.h

@@ -32,7 +32,7 @@ extern "C" {
 #include <spa/utils/defs.h>
 #include <spa/utils/list.h>
 
-/** \defgroup spa_interfaces SPA Interfaces
+/** \defgroup spa_interfaces Interfaces
  *
  * \brief Generic implementation of implementation-independent interfaces
  *
@@ -230,7 +230,7 @@ struct spa_interface {
  * \}
  */
 
-/** \defgroup spa_hooks SPA Hooks
+/** \defgroup spa_hooks Hooks
  *
  * A SPA Hook is a data structure to keep track of callbacks. It is similar to
  * the \ref spa_interfaces and typically used where an implementation allows

spa/include/spa/utils/json.h

@@ -37,7 +37,7 @@ extern "C" {
 
 #include <spa/utils/defs.h>
 
-/** \defgroup spa_json SPA JSON Parser
+/** \defgroup spa_json JSON
  */
 
 /**

spa/include/spa/utils/keys.h

@@ -29,7 +29,7 @@
 extern "C" {
 #endif
 
-/** \defgroup spa_keys  SPA Key Names
+/** \defgroup spa_keys  Key Names
  */
 
 /**

spa/include/spa/utils/list.h

@@ -30,7 +30,12 @@ extern "C" {
 #endif
 
 /**
- * \addtogroup spa_utils
+ * \defgroup spa_list List
+ *
+ */
+
+/**
+ * \addtogroup spa_list List
  * \{
  */
 

spa/include/spa/utils/names.h

@@ -29,7 +29,7 @@
 extern "C" {
 #endif
 
-/** \defgroup spa_names SPA Factory Names
+/** \defgroup spa_names Factory Names
  */
 
 /**

spa/include/spa/utils/result.h

@@ -30,7 +30,12 @@ extern "C" {
 #endif
 
 /**
- * \addtogroup spa_utils
+ * \defgroup spa_result Result handling
+ *
+ */
+
+/**
+ * \addtogroup spa_result
  * \{
  */
 

spa/include/spa/utils/ringbuffer.h

@@ -30,7 +30,12 @@ extern "C" {
 #endif
 
 /**
- * \addtogroup spa_utils
+ * \defgroup spa_ringbuffer Ringbuffer
+ *
+ */
+
+/**
+ * \addtogroup spa_ringbuffer
  * \{
  */
 

spa/include/spa/utils/string.h

@@ -36,7 +36,12 @@ extern "C" {
 #include <spa/utils/defs.h>
 
 /**
- * \addtogroup spa_utils
+ * \defgroup spa_string String handling
+ *
+ */
+
+/**
+ * \addtogroup spa_string
  * \{
  */
 
@@ -45,7 +50,6 @@ extern "C" {
  *
  * If both \a a and \a b are NULL, the two are considered equal.
  *
- * \ingroup spa
  */
 static inline bool spa_streq(const char *s1, const char *s2)
 {

spa/include/spa/utils/type.h

@@ -31,7 +31,7 @@ extern "C" {
 
 #include <spa/utils/defs.h>
 
-/** \defgroup spa_types SPA Types
+/** \defgroup spa_types Types
  */
 
 /**