Undo/Redo of sample editor

• 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).