editor: fix changing the proxy port

The "proxy port" spin button shared the same GTK adjustment with the
"connect timeout" one and so their values were always
synchronized. Fix this.

Fixes: 665ba502

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/issues/103
2 jobs for bg/proxy-port-adjustment in 24 minutes and 3 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #182464
fedora28_dist

00:16:07

 
  Test
passed #182465
fedora_from_dist

00:07:55