aFilter is useful for determining if a frame should output itself during
a List(). In any event, the children of the frame are always processed.
added nsIListFilter, an interface for any object that wants to act like a filter for frame dumps
added static NS_LAYOUT nsIListFilter * GetFilter(nsString *aFilterName);
currently, only TableListFilter implements nsIListFilter.
in response to List(...TableListFilter), the frame model is dumped in a way that is useful
for table layout regression testing. Currently this is controlled by command line switches on the viewer app.
table column balancing. Implemented in BasicTableLayoutStrategy (HTML4-spec).
nsTableFrame instantiates a strategy depending on the compatibility mode.
