Add utilities for layout with Cassowary

[ghost]

The most common usecase of this library, layout, is still kinda cumbersome with this library since one still needs to map all the values to and from the Cassowary variables. It would be nice to have sensible defaults and utilities to be able to write code like this:

import Aqt.Cassowary.Layout 1.0

Layout {
    id: root

    Constraint { expr: eq(button2.left, plus(button1.right, 20)) }
    Constraint { expr: eq(root.horizontalCenter, divide(plus(button1.left, button2.right), 2)) }

    Anchors {
        id: button1
        Button {}
    }

    Anchors {
        id: button2
        Button {}
    }
}