Remember last used file extension for saving new files

[bjorn]

Some formats support multiple files extensions (TMX supports .xml in addition to .tmx/.tsx), JSON supports .json in addition to the recently added .tmj/.tsj). When saving new files, the suggested file format is based on the last used one, but the file extension is always the first one (see Utils::firstExtension).

It would be nice if the last used file extension was also remembered, and suggested instead of the first one listed for the selected file format.

Split off from issue #3317.

[eishiya]

This probably also affects the object types file, which supports JSON and XML but lists them as one format. However, I think the better solution for that is probably to split the formats into separate options, as on MacOS the allowed extensions are not explicitly listed, making it hard to tell that you can use either format.

Edit: Object types are now split into two as of e41316a267b06e6bfcf835e9acd262242a8a748f