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/monitor/device.h

@@ -35,7 +35,7 @@ extern "C" {
 #include <spa/pod/event.h>
 
 /**
- * \defgroup spa_device SPA Device
+ * \defgroup spa_device Device
  *
  * The device interface can be used to monitor all kinds of devices
  * and create objects as a result. Objects a typically other

spa/include/spa/monitor/event.h

@@ -31,6 +31,11 @@ extern "C" {
 
 #include <spa/pod/event.h>
 
+/**
+ * \addtogroup spa_device
+ * \{
+ */
+
 /* object id of SPA_TYPE_EVENT_Device */
 enum spa_device_event {
 	SPA_DEVICE_EVENT_ObjectConfig,
@@ -47,6 +52,10 @@ enum spa_event_device {
 	SPA_EVENT_DEVICE_Props,		/* properties for an object (SPA_TYPE_OBJECT_Props) */
 };
 
+/**
+ * \}
+ */
+
 #ifdef __cplusplus
 }  /* extern "C" */
 #endif

spa/include/spa/monitor/type-info.h

@@ -33,6 +33,11 @@ extern "C" {
 
 #include <spa/monitor/event.h>
 
+/**
+ * \addtogroup spa_device
+ * \{
+ */
+
 #define SPA_TYPE_INFO_DeviceEvent		SPA_TYPE_INFO_EVENT_BASE "Device"
 #define SPA_TYPE_INFO_DEVICE_EVENT_BASE		SPA_TYPE_INFO_DeviceEvent ":"
 
@@ -51,6 +56,10 @@ static const struct spa_type_info spa_type_device_event[] = {
 	{ 0, 0, NULL, NULL },
 };
 
+/**
+ * \}
+ */
+
 #ifdef __cplusplus
 }  /* extern "C" */
 #endif

spa/include/spa/monitor/utils.h

@@ -32,6 +32,11 @@ extern "C" {
 #include <spa/pod/builder.h>
 #include <spa/monitor/device.h>
 
+/**
+ * \addtogroup spa_device
+ * \{
+ */
+
 struct spa_result_device_params_data {
 	struct spa_pod_builder *builder;
 	struct spa_result_device_params data;
@@ -90,6 +95,10 @@ static inline int spa_device_enum_params_sync(struct spa_device *device,
 #define spa_device_emit_event(hooks,e)		spa_device_emit(hooks,event, 0, e)
 #define spa_device_emit_object_info(hooks,id,i)	spa_device_emit(hooks,object_info, 0, id, i)
 
+/**
+ * \}
+ */
+
 #ifdef __cplusplus
 }  /* extern "C" */
 #endif