Screen language keywords (incl. at) take comma separated expressions.
Simple expressions take words as field names.

This lets one use a keyword (like transform) as a field name.
sl2: Fix regression in imagemap.

The _reuse() method would clear out the fixed during reuse,
which would remove the ground from the imagemap.
Allow ImageButtons to not have children.

Since ImageButtons do not use their child, it makes sense to not
require one - especially since _clear() will occasionally remove
it.