native/linux_x11: Allow setting WM_CLASS

[bolphen]

Following #501

Since Platform is Copy, I used &'static str instead of String for the name. Shouldn't be a problem cuz you won't want to change it.

Also fixed a typo in icon :)

[not-fl3]

Thanks for PR!
Interesting that this is the requirement for gnome, but if it works - it works, I guess!

[bolphen]

Yeah it's unfortunate that it doesn't use the X11 icon as a fallback