Add window icon support

[faiface]

Add a *pixelgl.Window method to set the icon of the window. The icon is usually displayed at the top bar of the window, or in the task bar of the desktop environment.

• (*pixelgl.Window).SetIcon(pixel.Picture) - sets the icon of the window to the given picture

I don't think a getter is necessary.

Also, make it possible to add an icon to pixelgl.WindowConfig by adding a Icon field there. Is the (*pixelgl.Window).SetIcon actually necessary with this field?

[otraore]

For most users, it probably wouldn't be necessary to have a SetIcon method but in some use cases, it might be useful. For example updating the icon to update the status of the game or something.

[faiface]

Yes, I also don't find that method to be particularly useful. It might be a good idea to leave it out, in order to avoid feature blow.