Fix extension points and Java auto-detect in Flatpak

[zocker-160]

Following changes have been made:

• replaced deprecated / EOL com.valvesoftware.Steam.Utility.gamescope extension
• added extension point for MangoHud
• moved ENV variables from starter script into manifest, this way PATH is set correctly also when I use a custom command inside the Flatpak
• fixed Java 8 appearing twice in the version selector auto-detect function thingy
• switched x11 socket to fallback-x11, since wayland is the primary socket to use when available

Known issue:

I have not checked the source code yet, but for some reason, the MangoHud check is greyed out despite mangohud being in PATH and confirmed working by adding mangohud --dlsym to Wrapper command.

Maybe someone knows how PollyMC is checking if mangohud is available or not? It is available in PATH:

$ flatpak run --command=bash org.fn2006.PollyMC 
[📦 org.fn2006.PollyMC ~]$ which mangohud
/app/utils/mangohud/bin/mangohud

UPDATE: Issue is fixed with move to 23.08. Users have to have MangoHud flatpak installed

flatpak install org.freedesktop.Platform.VulkanLayer.MangoHud//23.08

and then it will show up correctly in the settings: