ONBS doesn't recognize changes to tempo as an edit

[tehbigmac]

Description
When changing the tempo on a saved file (meaning there is mo asterisk indicating unsaved changes after the file name in the title bar), ONBS doesn't add an asterisk to indicate the file's current state has not been saved, nor does it prompt the user to save changes before closing.

To Reproduce

1. Open a preexisting file, or create and save a new one.
2. Set the tempo to a different value than the current one.
3. Notice how the asterisk does not appear next to the file name.
4. Additionally, close the program and observe how no "save changes before closing" prompt appears.

Expected behavior
When the tempo of a saved song is changed, it should prompt the program to recognize it as an unsaved file, just as if an edit was made to the notes themselves.

Additional context
ONBS recognizes modified song properties as unsaved changes to a song, so it's odd that tempo isn't recognized as such.