Modernize UIFont/NSFont

Our bindings for UIFont/NSFont are *old*, and that shows.

We can improve these bindings in numerous ways:

* Share the source code between these two types, the API is quite similar:
* Merge the api definitions and moving them into src/xkit.cs
* Merge src/AppKit/NSFont.cs and src/UIKit/UIFont.cs into a new src/XKit/XFont.cs
* Fix/enable nullability.
* Improve naming to our current standards:
* Methods should have a verb: `SystemFontOfSize` -> `CreateSystemFontOfSize`.
* Methods should not have unnecessary suffixes: `SystemFontOfSize` -> `SystemFont`.
* The actual method name in this case should be: `SystemFontOfSize` -> `CreateSystemFont`.
* Ensure that all static methods returning fonts return a new instance.
* Also add tests that calls these static methods and dispose the return value.
* This should also likely affect naming: these methods should use the `Create` verb (which implies that something got created), vs `Get` (which makes it sound like a lighter-weight property getter).
* In general try to have the same API in both UIFont and NSFont (for instance UIFont implements the == and != operators, while NSFont doesn't - this involves figuring out whether the UIFont operators are really necessary, if so, if the same applies to NSFont, and if so, implement the operators for NSFont as well).