r4296 index.php: speed up popup module by eliminating dependency on 320+ KB interface.php

r4296 index.php: speed up popup module by eliminating dependency on 320+ KB interface.php
navigation.php: replace function call with a static string, which will be overridden upon loading the config
interface.php: move $image array and 7 functions to interface-lib.php (new file)
init.php: require interface-lib.php unconditionally, gather require stements at top