Set App Icon from Pixels

[tonsky]

Follow up to #95

• [ ] Windows
• [ ] Linux
• [ ] macOS

[dzaima]

Currently looking into this, as part of trying to get general app icons for Linux. For that, there are, as far as I can tell, two approaches:

• have a globally-installed the-app.desktop file describing the program and its icon location (and the app setting a WM_CLASS hint to associate to it); seems impossible to make work via just setIcon, but would be good to have as a WindowX11-only thing for those wanting to do proper X11 application association;
• set via pixels; this could be bound to setIcon, but means something has to decode the png (or whatever file format).

I have both WM_CLASS configuring and icon-from-pixels configuring for X11 here via a temporary interface, using Skija to decode the png in the dashboard example. (the class isn't used by the example, but e.g. an OS-global icon theme could now set a custom icon for it)

[tonsky]

Both look good, make a PR?

I understand that setting icon is more of a packaging problem and probably can’t be easily solved by just doing API calls, but where it can work I don’t see why not have it