Add align_self property

[mhsmith]

We already support align_items, which aligns all of a box's children at once in the cross axis. We should also support align_self, which allows the alignment to be overridden on a per-child basis.

To avoid confusion, the existing alignment-related bugs should probably be fixed first (#2213).

Based on #3111, this property should have the alias horizontal_align or vertical_align when the parent is a column or row respectively. But this is complicated by the fact that during initialization, we don’t know what the parent will be yet.