António Fernandes authored Dec 10, 2023 and Peter Eisenmann committed Dec 11, 2023

GtkIconTheme changed behavior from GTK3 to GTK4. Before, it would
return NULL for a non-existent icon. Now it returns a non-scallable
`image-missing` icon.

But our icon info code still relies on NULL icon to set a fallback.

The documented solution to get the old behavior back is to call
gtk_icon_theme_has_gicon(). Let's just do that instead.

Also use  a more generic fallback icon that's been available in
adwaita-icon-theme for a few versions before our GTK4 port.

Closes #2796