Show file explorer when saving for the first time.

[SlaynAndKorpil]

Right now when you press Ctrl S you will never get a file selection as either you already have saved a to a file or the standard "save" (see ChessIO.scala) is used as a file name. It would be more intuitive to only use the last save file name when you already chose a file. So the first time you save you would always get the file chooser.