XML parser improvements

[rafern]

• Use warning/error system with XPaths to error sources in XML deserializer
• Get rid of BaseXMLUIParser
• Re-think value-passing
  • JSON values need to use quotes for keys, which is painful
  • Enums are non-existent. If you want centered body text, you have to pass 0.5 instead of TextAlignMode.Center, which is very confusing
  • Using a prefix in the value to represent how it should be parsed is stupid. Find a better alternative
  • There is no way to pass Infinity, which is a major pain when working with constraints
  • You should be able to mix variables and complex data structures. For example, passing a constraint with a maxWidth represented by a variable, like so: [0, maxWidth, 0, Infinity]
• Make parser versioned so that it's compatible with older format versions