More transparent file format choices in graphical interface

[tturocy]

In practice, the most important formats to be able to save games in would be .efg and .nfg. But we make this very hard - "save" actually defaults to the .gbt XML file format (which we will deprecate soon).

Move saving .efg and .nfg formats into "Save" from "Export" and tidy up/modernise the UX for saving files.