Duplicated labels for menu buttons

[hturner]

Comment from @ajrgodfrey

The screen reader reads out the text for each menu button twice, making it sound like there are two buttons (for both file and plot panes)

[ajrgodfrey]

This occurs when the button is greyed out or unavailable. And not when it is functional.

[georgestagg]

I wasn't able to reproduce this with macOS VoiceOver, but I'll try again with a different screen reader.