Qt capture interfaces dialogue string tweaks

To clarify that capturing always uses a file, the checkbox only controls whether
that file is temporary or persisted.

Change-Id: I939e1fdf488f53245166612193c60a366f63a18b
Reviewed-on: https://code.wireshark.org/review/4537
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>