Added 'XliffOrderedWalker' to support ordered execution pipeline of visitors. Visitors will be applied in the order they were added to the walker. It may be useful when next visitor execution depends on the results of execution of the previous visitor, i.e. 'unit visitor' execution may depend on changes done by 'segment visitor'.
Something about the abstraction here bothers me a little bit. With this change, IXliffWalkerStrategy implementations now need to implement both the "regular" and ordered doWalk methods. Shouldn't that distinction be encapsulated by the strategy itself?