Qt: keep "Copy to clipboard" button active when closing the capture_file_properties_dialog

As the Capture File Properties window content is kept once the capture is closed, allow to copy it

Change-Id: I11466e102fcf75a31aaa22225861ad8bf27f89a8
Reviewed-on: https://code.wireshark.org/review/5942
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>