WinUIpad: Code Cleanup, “Needs to Be Saved” Indicator, More ⭐

In this update, a little code cleanup, a "Need to be Saved" indicator, and new minimum app window width and height settings.

Remember, the latest version of the code is available on GitHub.
A little code cleanup
Given all the refactoring work I'd done on the WPF version of this app (.NETpad), I was never super comfortable with the quick and dirty approach I was taking to WinUIpad. But this was born of necessity: The Windows App SDK isn't just new to me, it's terrible, all kinds of terrible, and ...