REORG: applet: move the applet definitions out of stream_interface

We're tidying the definitions so that appctx lives on its own. A new
set of applet.h files has been added for this purpose.
diff --git a/include/proto/obj_type.h b/include/proto/obj_type.h
index a02e1d6..6d6577c 100644
--- a/include/proto/obj_type.h
+++ b/include/proto/obj_type.h
@@ -24,6 +24,7 @@
 
 #include <common/config.h>
 #include <common/memory.h>
+#include <types/applet.h>
 #include <types/connection.h>
 #include <types/listener.h>
 #include <types/obj_type.h>