Rename Elvish Castle terrain to "Elven Castle" for consistency with its keep

This is an editor-only change.
Use correct existing titlescreen icon in the Hotkey preferences dialog

The name in the code was icons/icon-title_screen.png, but LordBob
commited it as icons/icon-titlescreen.png instead. Since no other icon
in the directory uses underscores, let's stick to LordBob's spelling.