Improve guiIconText function using comptime so we don't have to cast enum to int - #161

I reckon we don't need the integer thing at all right? Having the enum would be enough?

I know this isn't all that relevant but your icon code could be greatly simplified to this:

    const icon: c_int = switch (@TypeOf(iconId)) {
        GuiIconName => @intCast(@intFromEnum(iconId)),
        else => @intCast(iconId),
    };