Plugins Save about tiled

[porfirioribeiro]

Instehead of having a MapReaderInterface and MapWriterInterface, tiled could have a MapSaveInterface (or better name) and MapExportInterface

Plugins using MapSaveInterface are plugins that support both saving and loading functions, that will appear on the Save and the Load dialogs and may be able to support more than one file type.
Plugins using MapExportInterface would just support save and appear on the Export dialog!

The Map class, should be able to know wish format it was loaded on so when we click save it would automatically use the plugin associated.

I think Tmx map support should follow the same rules and extend MapSaveInterface, so Tiled would not care about Tmx specifically as it does now, but just use it as any other Interface, it would just be the default one.