[DO NOT MERGE] Bleeding-edge Steam support

[MiaFoxcat]

I added very primitive Steam support by prompting the user whether they own Falcon 4.0 on GOG or Steam, and if they select Steam, the helper skips the GOG installer and then installs Steam, prompting the user to install F4.0 and run it once.

Currently there are two things that bug me a little but I unfortunately don't have the time nor bash knowledge to dig into them:

1. The helper has a "pausing" popup that the user has to not confirm until they've ran F4.0; would be better to find a way to wait until F4.0 is installed.
2. Steam is missing fonts - this can likely be fixed by adding the right font through winetricks, but I couldn't find which font Steam was using.

This PR is probably not production-ready, but it is something that is usable right now, albeit finicky. My hope was to provide a way that works and can be built upon.

[MaxWaldorf]

Sorry, this is not what I'm looking for with this script...

On top, you need F4 installed in the same wine install as BMS which your script doesn't do.

The plan is to check for F4 steam directory, copy the content over to the BMS install drive and create the proper missing registry keys.

Without an official F4, BMS won't start...

[MaxWaldorf]

As commented, this is not enough and not working for BMS run...

[MiaFoxcat]

Hi, it in fact does install in the same wine prefix. I literally tried it and it works.

[MiaFoxcat]

I changed the title to show it's not to be merged, and emphasized it's bleeding-edge, as well as marking it as draft. I think it's worth keeping until there's a better support in place, so that people can use something, albeit at their own risk.