Qt: Preserve capture filter when preferences changed

Qt: Preserve capture filter when preferences changed

Avoid that the last entry from recent.capture_filter is added to the
capture filter combo when editing preferences or changing profile.

This bug was introduced in gb7897dde.

Change-Id: I38a32386765c9e7ffaa93d006ff0ef7b78ac8252
Reviewed-on: https://code.wireshark.org/review/17453
Petri-Dish: Stig Bjørlykke <stig@bjorlykke.org>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Peter Wu <peter@lekensteyn.nl>
Reviewed-by: Anders Broman <a.broman58@gmail.com>