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.

sandboxes/orca_twistify_plugin_example_any.py

@@ -24,9 +24,9 @@ preview corkscrews and the print keeps correct walls/infill/flow.
 
 Because we edit geometry in place, surface types are preserved automatically
 (no per-surface type carry needed), and no numpy is required --
-rotate/scale/translate are host methods. Parameters come from ctx.params (the
-settings table above). The first object layer is untouched (z_rel = 0), so bed
-adhesion is unaffected.
+rotate/scale/translate are host methods. Parameters come from self.get_config()
+(see _params below), which the host seeds from get_default_config(). The first
+object layer is untouched (z_rel = 0), so bed adhesion is unaffected.
 """
 import math
 import json