Migrate autofill.h and autofill.w into IXSize

[brandonscript]

Seems unnecessary to have two disparate means of controlling height/width. Change autofill.h and autofill.w to simply be:

size.h: autofill size.w: autofill

[jeremyanticouni]

Autofill. I'd expect auto to size to the contents of the child controls, whereas autofill denotes that it will be filling the remaining space.