Upgrade to newer XamlX

[sowelipililimute]

Requires space-wizards/space-station-14#43866

This updates our XamlX to a newer version that lets us more easily support features such as templated controls and data binding without having to try and backport verifiable code fixes to our 5-year-old fork of XamlX.

Breaking Changes

The syntax for multiple style classes has changed from:

<Control.StyleClasses>
  <system:String>Hello</system:String>
  <system:String>World</system:String>
</Control.StyleClasses>

to

<Control StyleClasses="Hello World" />

[Fildrance]

New declaration style would be great improvement, very nice.

Having mandatory change to new style is meh to me, perosnally - i have hard time imagining where it would fit (maybe situation with extra-long style name, or having to combine 10+ styles? well person building it would be failing at design at that point i think).

Having nullable suppression in parts that are not 1-to-1 copes from xamlx could worth have proper null handling, but i don't exepct it to matter too much.