ui/layout: introduce Context

Context keeps the current Constraints and Dimensions so the layout
function scopes don't have to.

With

	ctx := new(layout.Context)

a label with margins and alignment goes from

	return al.Layout(ops, cs, func(cs layout.Constraints) layout.Dimensions {
		in := layout.Inset{...}
		return in.Layout(c, ops, cs, func(cs layout.Constraints) layout.Dimensions {
			return text.Label{...}.Layout(ops, cs)
		})
	})

to

	al.Layout(ops, ctx, func() {
		in := layout.Inset{...}
		in.Layout(c, ops, ctx, func() {
		       text.Label{...}.Layout(ops, ctx)
		})
	})

It was a difficult trade-off between the verbose functional approach
and the shorter but more complex Context.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

ui/layout/list.go

@@ -58,7 +58,7 @@ type List struct {
 
 // ListElement is a function that computes the dimensions of
 // a list element.
-type ListElement func(cs Constraints, index int) Dimensions
+type ListElement func(index int)
 
 type iterationDir uint8
 
@@ -96,12 +96,16 @@ func (l *List) init(cfg ui.Config, q input.Queue, ops *ui.Ops, cs Constraints, l
 }
 
 // Layout the List and return its dimensions.
-func (l *List) Layout(c ui.Config, q input.Queue, ops *ui.Ops, cs Constraints, len int, w ListElement) Dimensions {
+func (l *List) Layout(c ui.Config, q input.Queue, ops *ui.Ops, ctx *Context, len int, w ListElement) {
+	cs := ctx.Constraints
 	for l.init(c, q, ops, cs, len); l.more(); l.next() {
 		cs := axisConstraints(l.Axis, Constraint{Max: inf}, axisCrossConstraint(l.Axis, l.cs))
-		l.end(w(cs, l.index()))
+		i := l.index()
+		l.end(ctx.Layout(cs, func() {
+			w(i)
+		}))
 	}
-	return l.layout()
+	ctx.Dimensions = l.layout()
 }
 
 func (l *List) scrollToEnd() bool {