Add a new type 'byte' to be able to handle binary data while still
retaining the utf8 char and string behavior.
Internally the file and string port and character handling have
been reworked to use the new byte-centric functions.
Also adds a new test script (test9) that tests byte, char and
utf8 string handling.
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
