Add a utility program that prints out all the icon names in the gnome theme

hex-grid/src/bin/icon.rs

@@ -0,0 +1,8 @@
+use gtk::prelude::*;
+
+fn main() {
+    gtk::init();
+    for name in gtk::IconTheme::new().icon_names() {
+        println!("{}", name);
+    }
+}