Undo/redo functionality about audio-visualizer-python

[tassaron]

As of 22978a0, the following actions are undoable:

• Rename preset
• Delete preset
• Open preset (also triggered by saving a new preset)
• Clear preset
• Add component
• Remove component
• Move component (consecutive moves in the same direction are merged)
• Changing value of a widget in a component's settings page (consecutive changes to the same widget are merged)

Components may temporarily disable the merging of change actions as a cheap way to distinguish a memorable change from a minor change, e.g., clicking the [...] button vs. typing one character in a filepath.

Any component setting not represented by a widget must have a custom action defined, which is currently only used for the Conway's Game of Life component.

Opening a project or creating a new project clears the undo list.