layout: add NewContext, make zero value Contexts useful

While here, unexport the Queue and Config fields. The NewContext
cosntructor is shorter, and there is no reason to expose the fields
to accidental mutation.

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

widget/editor.go

@@ -114,7 +114,7 @@ func (e *Editor) processPointer(gtx *layout.Context) {
 		axis = gesture.Vertical
 		smin, smax = sbounds.Min.Y, sbounds.Max.Y
 	}
-	sdist := e.scroller.Scroll(gtx.Config, gtx.Queue, gtx.Now(), axis)
+	sdist := e.scroller.Scroll(gtx, gtx, gtx.Now(), axis)
 	var soff int
 	if e.SingleLine {
 		e.scrollRel(sdist, 0)