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/enum.go

@@ -37,7 +37,7 @@ func (e *Enum) Changed() bool {
 
 // Layout adds the event handler for key.
 func (e *Enum) Layout(gtx layout.Context, key string) layout.Dimensions {
-	defer op.Push(gtx.Ops).Pop()
+	defer op.Save(gtx.Ops).Load()
 	pointer.Rect(image.Rectangle{Max: gtx.Constraints.Min}).Add(gtx.Ops)
 
 	if index(e.values, key) == -1 {