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).

added GUI enhancement labels

requested review from @j.wuttke

mentioned in issue #55 (closed)

changed the description

OK. Can we please get an oral presentation of the hidden thing? Would you suggest times on Slack?

approved this merge request

merged

mentioned in commit b5c4961d