BASE: ENGINES: Change saveload code to adapt to the new MEC class.

- MEC: MetaEngineConnect.
- How do games handle save/load if MetaEngine (detection) is seperate from MetaEngineConnect (engine factory)?

- Most of the changes are quite similiar. ConfMan finds us the relevant MetaEngine, then simply use the new helpers from PluginMan.
- The new helpers will help convert a relevant MetaEngine into the other format or vice versa.
- Once the matching is complete, simply invoke functions by:
plugin->get<MetaEngineConnect>().engineMethod();
- Refer to previous commits to see the new class changes & notes.

gui/saveload.h

@@ -40,7 +40,7 @@ protected:
 	const U32String _buttonLabel;
 	const bool _saveMode;
 
-	void selectChooser(const MetaEngine &engine);
+	void selectChooser(const MetaEngineConnect &engine);
 public:
 	SaveLoadChooser(const U32String &title, const U32String &buttonLabel, bool saveMode);
 	~SaveLoadChooser();