Allow IP addresses with netmask in advanced rule editor

Especially for firewall configuration, it's very much useful to be
able to allow or block subnets (e.g. I want to allow all traffic
originating on trusted part of my home network). Both UFW and
firewalld do accept IP addresses with netmask, so it is just about
allowing them in the input's validtors.

I decided to replace the regular expressions with a custom QValidator
subclass, which uses lib'c inet_pton() to validate the entered IP
address and can also handle optional netmasks.

FEATURE: 434317
CHANGELOG: Allow IP addresses with netmask in advanced rule editor