[kernel] add user directories

configurator.ml

src/kernel_internals/runtime/macos_dirs.ml

src/kernel_internals/runtime/system_config.ml.in

src/kernel_internals/runtime/system_config.mli

src/kernel_internals/runtime/unix_dirs.ml

src/kernel_internals/runtime/win_dirs.ml

src/kernel_services/plugin_entry_points/kernel.ml

src/kernel_services/plugin_entry_points/kernel.mli

src/kernel_services/plugin_entry_points/plugin.ml

src/kernel_services/plugin_entry_points/plugin.mli

src/plugins/gui/gtk_helper.ml

src/plugins/gui/gui_parameters.ml

src/plugins/gui/gui_parameters.mli