Fix plugin configuration not taking effect, and improve the plugin config UI (#14944)

# Description

Changing a slicing plugin's configuration had no effect on the sliced
result until you forced a re-slice some other way; it now applies
immediately. Print, printer and filament presets also keep their plugin
configuration separately, so configuring a plugin on one no longer wipes
out what you set on another.

A plugin's custom configuration page gets the same round of improvements
in both the Plugins dialog and the per-preset dialog: it follows the
app's light/dark theme, keeps its state while you edit instead of
resetting under the cursor, and can tell whether it is being edited
globally or for a preset, so "Restore defaults" can be labeled for what
it will actually do. The two bundled examples show this off — Twistify
now ships a custom configuration UI, and Inspector is themed, groups

# Screenshots/Recordings/Graphs



https://github.com/user-attachments/assets/02ca062a-5143-49a3-abe0-a2a040b3a928



## Tests

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src/libslic3r/Preset.hpp

@@ -407,6 +407,11 @@ public:
     // Printer machine limits, those are contained in printer_options().
     static const std::vector<std::string>&  machine_limits_options();
 
+    // Option key holding this preset type's plugin capability overrides. Each type has its own key so
+    // the values survive the merge into a single full config; print is the fallback for the types with
+    // no plugin-backed options.
+    static const char*                      plugin_overrides_key(Type type);
+
     static const std::vector<std::string>&  sla_printer_options();
     static const std::vector<std::string>&  sla_material_options();
     static const std::vector<std::string>&  sla_print_options();