Name the plugin capability config option consistently

The preset option was plugin_preference_overrides while the GUI field type
that renders it was GUIType::plugin_config, for one and the same thing.
Settle on "config": the store, the dialog and the Python hooks all say config
already, so renaming that way touches 4 files instead of the whole plugin
subsystem and the public plugin API.

Keep the _overrides suffix — the option is the preset's override layer over
the base PluginConfig store, a distinction EffectiveCapabilityConfig tracks.

Also wrap the printer tab's group heading in L(); it was the only one of the
three missing it, and was therefore untranslatable.

src/slic3r/plugin/PluginConfig.hpp

@@ -44,7 +44,7 @@ private:
     std::vector<nlohmann::json>                  m_opaque_entries;
 };
 
-inline constexpr const char* PLUGIN_OVERRIDES_OPTION_KEY = "plugin_preference_overrides";
+inline constexpr const char* PLUGIN_OVERRIDES_OPTION_KEY = "plugin_config_overrides";
 
 std::string plugin_overrides_of(const Preset& preset);
 bool parse_plugin_overrides(const std::string& raw, CapabilityConfigDocument& document, std::string& error);