setting proxy in network settings should warn when overwriting apt proxy in /etc/apt/apt.conf

[Karl-Philipp Richter]

Setting a system-wide HTTP-/HTTPS-/FTP- or SOCKS-proxy in the network settings overwrites entries in `/etc/apt/apt.conf` (maybe in `/etc/apt/apt.conf.d/` as well. The user should be warned if the files have been changed manually and contain a proxy setting.

ProblemType: Bug
DistroRelease: Ubuntu 14.10
Package: ubuntu-system-settings 0.3+14.10.20141022-0ubuntu1
Uname: Linux 3.19.0-031900rc3-generic x86_64
ApportVersion: 2.14.7-0ubuntu8
Architecture: amd64
CurrentDesktop: Unity
Date: Fri Jan 9 18:37:15 2015
EcryptfsInUse: Yes
InstallationDate: Installed on 2015-01-03 (6 days ago)
InstallationMedia: Ubuntu 14.10 "Utopic Unicorn" - Release amd64 (20141022.1)
SourcePackage: ubuntu-system-settings
SystemImageInfo:
 current build number: 0
 device name:
 channel: daily
 last update: Unknown
UpgradeStatus: No upgrade log present (probably fresh install)