ScriptFu: build shared libgimp-scriptfu-3.0.so separate from executable script-fu

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

plug-ins/script-fu/libscriptfu/script-fu-lib.h

@@ -0,0 +1,44 @@
+/* GIMP - The GNU Image Manipulation Program
+ * Copyright (C) 1995 Spencer Kimball and Peter Mattis
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+#ifndef __SCRIPT_FU_LIB_H__
+#define __SCRIPT_FU_LIB_H__
+
+gboolean     script_fu_extension_is_busy         (void);
+
+void         script_fu_find_and_register_scripts (GimpPlugIn     *plugin,
+                                                  GList          *paths);
+
+void         script_fu_set_run_mode              (GimpRunMode     run_mode);
+void         script_fu_init_embedded_interpreter (GList          *paths,
+                                                  gboolean        allow_register,
+                                                  GimpRunMode     run_mode);
+
+void         script_fu_set_print_flag            (gboolean        should_print);
+void         script_fu_redirect_output_to_gstr   (GString        *output);
+void         script_fu_redirect_output_to_stdout (void);
+void         script_fu_print_welcome             (void);
+
+gboolean     script_fu_interpret_string          (const gchar     *text);
+const gchar *script_fu_get_success_msg           (void);
+
+void         script_fu_run_read_eval_print_loop  (void);
+
+void         script_fu_register_quit_callback         (void (*func) (void));
+void         script_fu_register_post_command_callback (void (*func) (void));
+
+#endif /* __SCRIPT_FU_LIB_H__ */