Complete the plugin settings removal after the feat/plugin-feature merge

The merge kept this branch's PluginConfig design, which deletes
PluginDescriptor::settings, get_plugin_settings() and ctx.params, but left
references to them behind: the slic3rutils target did not build, and the
bindings test still asserted the removed ctx.params attribute.

Port the two settings tests onto PluginConfig instead of dropping them. They
guard a field bug where a cloud-metadata refresh wiped a plugin's settings and
it silently ran on its own defaults, so the equivalent properties are still
worth pinning: that a stored config survives the refresh, and that an edited
config reaches the plugin through a real dispatch.

Also defer PluginsConfigDialog's web commands off the webview script-message
callback, as PluginsDialog already does. Its remove_preset_override handler put
a modal wxMessageBox on that stack, which is the GTK crash class fixed in
b779a7bfed/f2ccbfc8b5 for the sibling dialog.

tests/slic3rutils/CMakeLists.txt

@@ -8,7 +8,7 @@ add_executable(${_TEST_NAME}_tests
     test_plugin_install.cpp
     test_plugin_lifecycle.cpp
     test_slicing_pipeline_bindings.cpp
-    test_slicing_pipeline_params.cpp
+    test_slicing_pipeline_config.cpp
     test_plugin_sort.cpp
     test_plugin_cloud_metadata.cpp
     ../fff_print/test_helpers.cpp