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.
Why:
1) users can install .scm scripts to plug-ins dir
2) Crashing scripts do not crash extension-script-fu
Scripts (.scm files) have a shebang and are executable
and in a same-named subdir of plugin dir.
Interpreter/scripts create PDB procs of type PLUGIN unlike extension-script-fu
which creates PDB procs of type TEMPORARY, owned by extension-script-fu.
Unlike other interpreters, the interpreter is-a plugin outright,
not by virtue of the script subclassing GimpPlugin and using GI.
More details in /plug-ins/script-fu/interpreter/README
