Allow aligning of a view to a group of views

[carranca]

This is something accomplished using Barrier in ConstraintLayout, but I think we can make it more straightforward.

Given views A, B, C:

Scenarios

1. Aligning a view A to at any given time be to the left of both B and C
2. Aligning a view A to at any given time be to the right of both B and C
3. Aligning a view A to at any given time be horizontally centered to the middle point between B and C
4. Aligning a view A to at any given time be above both B and C
5. Aligning a view A to at any given time be below both B and C
6. Aligning a view A to at any given time be vertically centered to the middle point between B and C

When the layout of B or C is updated, the positioning of A must be updated.

[oldergod]

Can't we do that already?

[jamolkhon]

I'm pretty sure this is (now) supported.

1. x = leftOf { minOf(B.left(), C.left()) }
2. x = rightOf { maxOf(B.right(), C.right()) }
3. x = centerHorizontallyTo { XInt(abs(B.left().value - A.right().value)) }
4. etc