Migrate WarFoundryXmlFactory to XPath

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The initial version of WarFoundryXmlFactory loaded by walking the DOM very strictly. This is brittle to changes in tag order and would be better handled using XPath queries or similar.

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(In r20) Re #11 - Documentation

• Document Stats and SystemStats at class level to explain the difference
• Document WarFoundryXmlFactory at class level

Re #13 - XPath for XML loading

• Load categories using XPath - needs fixing so we can do a proper "categories/cat" query

Re #10 - Refactoring for readability

• Fix method signature for getting number from attribute added in r19
• Refactor more code in to GetExtraData method
• Separate out loading of categories for GameSystem

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(In r82) Re #13 - Use XPath for file loading

• Replace loading of race unit types, categories and equipment with XPath method
• Add loading of abilities using XPath

Also:

• Add implementation for Ability
• Add AddAbility method to Race
• Alter some logging so that "for ID" is followed by ID not name

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(In r88) Closes #13 - Migrate to XPath

• Remove elem.ChildNodes calls and use XPath to make sure we're getting the nodes we think we should

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Mark released changes as released

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Mark released fixes as released using radio values

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Mark fix as released under a previous version