feat: providing an optional render when an icon fails to resolve entirely

[mattkae]

What's new?

• If an icon fails to resolve, the caller can provide an option render function to use in its place

Motivation

We might want to show some sort of nice "Icon not found" icon when it is unavailable on a system for any reason.

Example usage

return XdgIcon(
        name: iconName,
        size: size,
        iconNotFoundBuilder: () {
          return SizedBox(
              width: size.toDouble(),
              height: size.toDouble(),
              child: Center(
                  child: Text(
                      iconName.isEmpty ? "U" : iconName[0].toUpperCase(),
                      style: Theme.of(context).textTheme.displayLarge)));
        });

[mattkae]

Note that the introduction of the new _iconNotFound boolean is to distinguish between the initial "loading" case and the "failure to resolve" case. Perhaps we would be better off with an initialLoading flag instead, but let me know

[spydon]

@mattkae all commits need to be signed before this can be merged.

[mattkae]

Eh hold on, I'll reopen this, since I think I did the wrong force push ;)