Rework and partially revert r33631.

Rework and partially revert r33631.

This patch introduces a "ListCommand" tag for Float layout. This tag
defines the command used to generate a list of such floats, in the cases
where float.sty is not used (i.e., where NeedsFloatPkg is true). We were
previously hardcoding the commands \listoftables and \listoffigures. But
in other cases, such as achemso.layout, which defines lists of schemes,
charts, etc, we could not output such lists.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@33633 a592a061-630c-0410-9148-cb99ea01b6c8