Update UI to show if there are unsaved changes.

[Kangaroux]

📖 Overview

• Save menu action should be enabled only when there are unsaved changes
• The window title should include an asterisk to indicate if the project is unsaved

A project has unsaved changes if:

1. The project is not yet saved to the disk.
2. OR There are pending changes to the project.
3. OR There are pending changes to any charts within the project.

A chart has unsaved changes if:

1. The chart has not yet been saved to the disk.
2. OR The chart's undo stack is non-empty
3. AND The item at the top of the undo stack is not the same item since the last save