layout: change Widget to take explicit Context and return explicit Dimensions

Change the definition of Widget from the implicit

        type Widget func()

to the explicit functional

        type Widget func(gtx layout.Context) layout.Dimensions

The advantages are numerous:

- Clearer connection between the incoming context and the output dimensions.
- Returning the Dimensions are impossible to omit.
- Contexts passed by value, so its fields can be exported
and freely mutated by the program.

The only disadvantage is the longer function literals and the many "returns".
What tipped the scales in favour of the explicit Widget variant is that type
aliases can dramatically shorten the literals:

	type (
		C = layout.Context
		D = layout.Dimensions
	)

	widget := func(gtx C) D {
		...
	}

Note that the aliases are not part of the Gio API and it is up to each user
whether they want to use them.

Finally the Go proposal for lightweight function literals,
https://github.com/golang/go/issues/21498, may remove the disadvantage
completely in future.

Context becomes a plain struct with only public fields, and its Reset is
replaced by a NewContext convenience constructor.

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

layout/list.go

@@ -29,7 +29,7 @@ type List struct {
 	// Alignment is the cross axis alignment of list elements.
 	Alignment Alignment
 
-	ctx         *Context
+	ctx         Context
 	macro       op.MacroOp
 	child       op.MacroOp
 	scroll      gesture.Scroll
@@ -51,7 +51,7 @@ type List struct {
 
 // ListElement is a function that computes the dimensions of
 // a list element.
-type ListElement func(index int)
+type ListElement func(gtx Context, index int) Dimensions
 
 type iterationDir uint8
 
@@ -82,7 +82,7 @@ const (
 const inf = 1e6
 
 // init prepares the list for iterating through its children with next.
-func (l *List) init(gtx *Context, len int) {
+func (l *List) init(gtx Context, len int) {
 	if l.more() {
 		panic("unfinished child")
 	}
@@ -100,16 +100,15 @@ func (l *List) init(gtx *Context, len int) {
 }
 
 // Layout the List.
-func (l *List) Layout(gtx *Context, len int, w ListElement) {
+func (l *List) Layout(gtx Context, len int, w ListElement) Dimensions {
 	for l.init(gtx, len); l.more(); l.next() {
 		crossMin, crossMax := axisCrossConstraint(l.Axis, l.ctx.Constraints)
 		cs := axisConstraints(l.Axis, 0, inf, crossMin, crossMax)
 		i := l.index()
-		l.end(ctxLayout(gtx, cs, func() {
-			w(i)
-		}))
+		gtx.Constraints = cs
+		l.end(w(gtx, i))
 	}
-	gtx.Dimensions = l.layout()
+	return l.layout()
 }
 
 func (l *List) scrollToEnd() bool {