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.

src/slic3r/GUI/PostProcessor.cpp

@@ -270,9 +270,6 @@ static void run_post_process_plugins(const ConfigOptionStrings& capabilities,
         ctx.host         = host;
         ctx.output_name  = output_name;
         ctx.full_config  = &config;   // no live Print here; config_value() reads this
-        // Hand the plugin its own [tool.orcaslicer.plugin.settings] as ctx.params (same plugin_key the
-        // capability was resolved by), mirroring the in-pipeline dispatcher in GUI_App.cpp.
-        const std::string plugin_key = ref.uuid.empty() ? ref.name : ref.uuid;
 
         ExecutionResult exec_result;
         try {