Recommend unique filename for new/default score

[channingko-madden]

Resolves: #20140

In the special case of a newly created score that has a default title, and there already exists an "Untitled\ score.mscz" file in the chosen save directory, recommend a different file name when saving locally (ex. "Untitled\ score (1).mscz") to prevent accidental overwriting.

This issue was brought up by a user who accidentally saved over an existing file, even though there is usually an overwrite dialog warning raised by the OS in this scenario, but thought MuseScore would number the new file in an intelligent way.

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