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/test_slicing_pipeline_bindings.cpp

@@ -106,7 +106,7 @@ TEST_CASE("orca.slicing is workflow-only: context exposes raw print/object; view
     py::object slicing = orca.attr("slicing");
 
     // Context surface: raw graph entry points + workflow accessors.
-    for (const char* name : { "print", "object", "params", "config_value", "cancelled",
+    for (const char* name : { "print", "object", "config_value", "cancelled",
                               "orca_version", "step" })
         CHECK(py::hasattr(slicing.attr("SlicingPipelineContext"), name));