CSSFrameConstructor/Constraints

Some constraints to start things off. Feel free to come up with a better syntax to mark optional and multiple frames

Tables

A table is always constructed of a frame tree like:

 nsTableOuterFrame
   (nsTableCaptionFrame)       optional
   nsTableFrame
     (nsTableColGroupFrame     optional, right?
       nsTableColFrame)
     nsTableRowGroupFrame      (is this optional?)
       nsTableRowFrame
         nsTableCellFrame

The nsTableCaptionFrame and nsTableCellFrames will hold non-table frames.