Qt: Enable/disable Capture Interfaces buttons consistently.

Qt: Enable/disable Capture Interfaces buttons consistently.

The "Compile BPFs" and "Start" buttons need to be enabled and disabled
at the same time and according to the same criteria. Make sure we do so.

Change-Id: I708886564dcf569cd71f936119195209ef6dcbd5
Reviewed-on: https://code.wireshark.org/review/20265
Petri-Dish: Gerald Combs <gerald@wireshark.org>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Jaap Keuter <jaap.keuter@xs4all.nl>
Reviewed-by: Gerald Combs <gerald@wireshark.org>