Qt: Capture fixes.

Don't check syntax when we rebuild the CaptureFilterCombo.

Make sure we switch back to the main window when capturing fails.

Change-Id: Id4137e33a8135645f29ca7b2836c76b90f09e3ac
Reviewed-on: https://code.wireshark.org/review/4311
Reviewed-by: Gerald Combs <gerald@wireshark.org>