add an undo/redo command for adding a layer in the sample editor.
hide undo/redo actions since undo/redo is not completed yet
minor refactorings
Adding one more undo/redo command has also the aim to show how to continue implementation of Undo/Redo. I edited issue #55 (closed) about this. This MR and especially commit 61e09de1 will be mentioned in this issue as a how-to.
I also commented the current state of implementation in issue #55 (closed).
