project_panel: Allow copying the paths of multiple selected files at once

[kjzl]

Closes #16555

Release Notes:

• Improved the "Copy Path" and "Copy Relative Path" actions in the project panel's context menu when selecting multiple files. All selected files' paths will now be copied, separated by newlines.

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[kjzl]

@cla-bot check

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[mrnugget]

Tested locally and works fine for me. Thank you!!