Undo/redo functionality

[tassaron]

Undo/redo functionality is essential for an editing program of any kind. Qt has an Undo Framework that seems straightforward to use, but it requires changing every potential user action into a QUndoCommand object and giving it a corresponding undo method. So this feature will require a lot of code changes.

The undo stack will only be used for the main window and component widgets. Components should be easy to convert by changing code in one place (component.py). The biggest thing is changing the component list actions and preset actions in mainwindow.py and presetmanager.py.

Hotkeys should be the standard Ctrl-Z for undo and Ctrl-Shift-Z/Ctrl-Y for redo. There could be undo/redo buttons next to the Projects button in the GUI.

[tassaron]

As of 22978a0635e906f8ebfa1de81cdc38fd96aad756, 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.