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
Changes are mostly to the dir structures and build system for ScriptFu.
Some changes to the outer plugin source to call the library.
Why: so that other executables (future gimp-scheme-interpreter,
or a future separated script-fu-server) can exist in separate directories,
and share the library in memory (when built shared.)
Whether the library is built shared and installed on its own
(versus static and not installed)
is a compile time option (both automake LibTool and meson abstract it away)
The default is shared and installed, say as libgimp-scriptfu-3.0.so.
Installed alongside other shared libraries (e.g. wherever libgimp is installed)
to simplify packaging.
A preliminary refactoring which helps enable MR gimp!647
