Using Icon Fonts with gui::Application: example requested

I see some docs for `FondDescription::AddTypefaceForCodePoints`, but I can't seem to get any icons from fontawesome to render; this is my code so far (assuming I've downloaded fontawesome regular to `/usr/share/fonts/fa-regular-400.ttf`):
````
auto& app = gui::Application::GetInstance();
app.Initialize(argc, argv);
auto default_font = app.GetFontDescriptions()[0];
default_font.AddTypefaceForCodePoints("/usr/share/fonts/fa-regular-400.ttf", std::vector{0xf15b});
app.SetFont(gui::Application::DEFAULT_FONT_ID, default_font);
app.AddWindow(std::make_shared());
app.Run();
````
(There are no errors in loading the font)
In the code for `MyWindow`, I'm trying to create a button to use an icon; I've tried the following, both of which result in a button with `?` as its text instead of the specified icon...
````
auto button = std::make_shared(u8"\uf15b");
...
auto button = std::make_shared("\xef\x85\x9b");
````

I'm not sure what I'm doing wrong here; any help/suggestions are much appreciated.
On that same note, it would be awesome if one of the sample apps used some font-based icons to demonstrate proper usage.
Thanks for this great library!