op: rename StackOp/Push/Pop to StateOp/Save/Load

The semantics were relaxed in a previous commit; this change renames
to operations accordingly.

API change. Use gofmt to adjust your code accordingly:

gofmt -r 'op.Push(a).Pop() -> op.Save(a).Load()'
gofmt -r 'op.Push(a) -> op.Save(a)'
gofmt -r 'v.Pop() -> v.Load()'
gofmt -r 'op.StackOp -> op.StateOp'

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

widget/material/editor.go

@@ -40,7 +40,7 @@ func Editor(th *Theme, editor *widget.Editor, hint string) EditorStyle {
 }
 
 func (e EditorStyle) Layout(gtx layout.Context) layout.Dimensions {
-	defer op.Push(gtx.Ops).Pop()
+	defer op.Save(gtx.Ops).Load()
 	macro := op.Record(gtx.Ops)
 	paint.ColorOp{Color: e.HintColor}.Add(gtx.Ops)
 	var maxlines int