Make interface types a string

This is more in line with wayland and it allows us to create new
interfaces in modules without having to add anything to the type
enum. It also removes some lookups to map type_id to readable
name in debug.

spa/include/spa/support/system.h

@@ -37,6 +37,9 @@ extern "C" {
 /**
  * a collection of core system functions
  */
+#define SPA_TYPE_INTERFACE_System	SPA_TYPE_INFO_INTERFACE_BASE "System"
+#define SPA_TYPE_INTERFACE_DataSystem	SPA_TYPE_INFO_INTERFACE_BASE "DataSystem"
+
 #define SPA_VERSION_SYSTEM		0
 struct spa_system { struct spa_interface iface; };