Resurrect XML actor support when QuirksMode=1

This patchset re-adds support for XML actors (à la ITG-style fgchange gimmicks).  Our club has been testing it weekly on a build from October with no major problems.  I just forward-ported the commits, compiled successfully, and hit it with my customary XML trial-by-fire, [Box](http://divinelegy.com/simfiles/zetorux/D.O.W.N.S.%204%20Individuals/Box.zip).  Played beautifully.

This would be a candidate for 5.1 if we decide to add such a milestone, but it could also be merged for 5.0 given two considerations: we've been testing it here for months, and it has no effect when QuirksMode is off (the default).

Commits 3ac15cc through da5eb84 could be generally useful as fixes or compat features in the 5.0 timeframe.  If you'd like, I can split them out.