XamlX upgrade

[sowelipililimute]

Mostly complete, besides one glaring BC break that I'm still ironing out, the deal with style classes relying on behaviour incinerated in kekekeks/XamlX@22c9930. There's a few ways that this could be tackled:

• push through the BC to content (infeasible given that style classes are relied on for things as basic as button styling and any user overriding them would need to explicitly write those out)
• duplicate the relevant transformer into our own repo to preserve the old behaviour (doesn't really let us implement new desired syntax)
• fully custom syntax for style classes (pain)
• making the control styleclass setter concatenate instead of overwriting (evil)

Opinions welcome from anyone with thoughts on the matter