User flow on launching GUI without a game loaded

[tturocy]

This is WIP towards a solution to #80.

Rather than re-using a frame as suggested by #80, this proposes a bit more modern style, whereby if a game is not loaded on the command-line (or otherwise at launch), there's a "welcome" frame that offers the ability to load a game, or to create a new one. This "welcome" frame is then dismissed after a game is loaded. So, this has roughly the same effect as intended by #80, although the mechanism is slightly different.

I would like to get views on:

• Does this work well / look decent on other platforms (I've only looked at MacOS);
• Any suggestions for improvements, or things that don't quite work right (try to break it!)