Add an icon browser

This is meant to help app authors choose the right icons.

demos/icon-browser/main.c

@@ -0,0 +1,8 @@
+#include <gtk/gtk.h>
+#include <iconbrowserapp.h>
+
+int
+main (int argc, char *argv[])
+{
+  return g_application_run (G_APPLICATION (icon_browser_app_new ()), argc, argv);
+}