Add remove button for cover art in export dialog

[Revisto]

Is your feature request related to a problem? Please describe.
There's no way to remove selected cover art in the export dialog. Users must close and reopen the dialog to clear it.

Describe the solution you'd like
Add a "Remove" button next to "Choose File" that clears the selected cover art and only shows when cover art is selected.

[Im-BlackDevil]

Hi, I’m new to contributing to GNOME projects and I’d like to work on this issue.

I am thinking like i will first locate the export dialog UI file and then add a conditional “Remove” button and clear the selected cover art state when clicked.

Does this approach sound correct?