Add Steam-Metadata-Editor support

[frostworx]

Recently I found the very useful tool Steam-Metadata-Editor
I think it would be nice feature to have direct support for it in steamtinkerlaunch.

[frostworx]

corresponding upstream issue

[frostworx]

I will mostly stop working on steamtinkerlaunch soon and only occasionally add one or another feature.
This project is still interesting, so leaving the issue open

[sonic2kk]

There has been no change on this upstream, but this would still be pretty nifty to have, so leaving this issue open 😄

Upstream is still interested too, no ETA (though not planned for next release). Leaving this open until this feature is implemented :-)

[sonic2kk]

I am not sure how I wanted to integrate this at the time. The maintainer of the project is really friendly, and I encourage anyone reading to check it out, but I think now there is no need for any integration :-)

Closing.