Settings UI

[laanwj]

For windowed operating systems it would be nice to have a little GUI where it's possible to configure the renderer, where to find the disk image (though, #24 may be better for that), and so forth. A bit like what ScummVM has. I don't particularly want a dependency on any widget toolkit like gtk or qt, though. So I'm not sure.
(one possibility is to use imgui here as we already use it for the optional debug UI)

This also needs the functionality to store persistent settings in an ini file or such.