Shortcut icons & bugfixes

Added shortcut icon and touch icons if present: Images must be stored
in (child)theme-folder/images/. Graceful fallback if 1 or more images
are not present. Image file names are hard-coded to: favicon.ico,
apple-touch-icon.png, apple-touch-icon-57x57-precomposed.png,
apple-touch-icon-72x72-precomposed.png,
apple-touch-icon-114x114-precomposed.png,
apple-touch-icon-144x144-precomposed.png