Apply a bodge to arrange that if the user selects Custom from the game

Apply a bodge to arrange that if the user selects Custom from the game
type dropdown, we still get an 'onchange' event if they select it a
second time. Normally this wouldn't happen, because onchange means
what it says and we only get it if a _different_ element is selected.

My solution is to create two list items called Custom, set one of them
as display:none to stop it showing up when the list is dropped down,
and to select it after the configuration box closes.

[originally from svn r9788]