Option to use Flatpaks under Linux

[RossBrunton]

This adds a configuration setting "linuxUseFlatpak" which controls whether Steam should be launched as a Flatpak or not. By default this is detected automatically based on the path, but an option in the settings can override the behaviour.

If this option is enabled, we first check if we are able to read from our config directory. If not, we display an error and ask the user to give us permission. If we do, then we launch Steam through flatpak rather than the binary directly.

Closes: #1144, #1051

[CLAassistant]

All committers have signed the CLA.

[RossBrunton]

So I did all this work, and then decided to check if someone else had done it, and it seems both @wash2 ( #798 ) and @pedroVH ( #1064 ) have done this earlier. Guess I should have checked first.

Anyway, I think hopefully this should be more sophisticated, since it doesn't assume any hardcoded paths, allows the user to configure things and shows up an error if it isn't configured correctly.

Also, full disclosure, I've only tested this on Lethal Company, and I'm super unfamiliar with typescript.

[RossBrunton]

Is there anything blocking this from being accepted?

[gegoxaren]

Why has this not been merged yet?

[ebkr]

Apologies for the huge delay on this.

The reason it took so long was two-fold.

1. I didn't want to support Flatpak Steam without also having a Flatpak build of the manager. It's pretty intuitive that both go together. The next release will make that properly available.
2. I don't have a Flatpak Steam installation to test this easily and it sucks having to ruin my current installation just to do so. I'll obviously take the hit at some point but this will still be further delayed for a little bit as I don't have a huge amount of storage space to dual boot.