ScriptFu: script-fu-register-filter for GimpImageProcedure.

Resolves #8382

Also v2 scripts infer and set sensitivity to drawables

Add two test plugins clothify-v3.scm and test-sphere-v3.scm.
Temporary, to be removed when 3.0 ships.

Some refactoring (extracting methods, moving functions to new files).

Some drive-by fixes to script-fu-arg.c revealed by using GimpProcedureDialog.

plug-ins/script-fu/libscriptfu/script-fu-interface.c

@@ -28,7 +28,6 @@
 #include <windows.h>
 #endif
 
-#include "tinyscheme/scheme-private.h"
 #include "scheme-wrapper.h"
 
 #include "script-fu-types.h"