Revision 198208

This patch implements a pluggable LayoutManagers system, according
to the ideas of Finn Bock and his patch in bug 30500.

Created a LayoutManagerMaker interface, with method
makeLayoutManagers(FONode, List), and two convenience methods on top
of it.

Created an implementation: LayoutManagerMapping. This is along the
pattern of FOElementMapping. It creates a Map from FObj class to Maker
objects. There are many static inner classes which are subclasses of
Maker. Each subclass implements its own version of the make method.

FOUserAgent has a setter and getter for
LayoutManagerMakerOverride. AreaTreeHandler creates a
LayoutManagerMaker, taking the user's override into account, using
LayoutManagerMapping as the default. It has a get method for it.

The LayoutManager interface has a get method for the AreaTreeHandler
object, a reference to which is held in PageSequenceLM, the top of the
LM tree.