The 'mutable' keyword exists for this kind of use.

Change-Id: Ic52192f53039b7f1b4529a93c13de74c5f54e7f8
Store the handler as a const pointer.

Handlers are conceptually stateless; we can keep them as const objects.

Change-Id: I3d35ab0695037632e82f67f901c9795adf9bdd8d
pImplize XMLPropertyHandlerFactory.

Change-Id: Iff524c9065e9374b39db998e3860704555130995