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/src/stream.c b/src/stream.c
index e9a4b0d..ed97e48 100644
--- a/src/stream.c
+++ b/src/stream.c
@@ -19,6 +19,7 @@
 #include <common/debug.h>
 #include <common/memory.h>
 
+#include <types/applet.h>
 #include <types/capture.h>
 #include <types/global.h>