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 !647