Support undo/redo

Undo/Redo implementation is started (in Layer oriented sample editor). It's not finished so far, therefore the undo/redo buttons are hidden right now.

The current state is [23 Feb 2022]:

• Undo/Redo commands are located in SampleEditorCommands.h and SampleEditorCommands.cpp
• These commands have to be called from SampleEditorController.
• Currently available commands are
  • Undo/Redo changing a double value (CommandChangeValue)
  • Undo/Redo adding a layer (CommandAddLayer)
  • Undo/Redo deletion of a layer (CommandRemoveLayer)
• Undo/Redo buttons are always hidden by code in LayerOrientedSampleEditor::updateActionEnabling()

To continue implementation please study the current implementation for understanding the principles like usage of the "path" of a DoubleDescriptor etc.

You can also study commit e10ae899 (contained in merge request !714 (merged)) to see how CommandAddLayer was added (how the original non-undo/redo code was modified to support undo/redo).