This adds a new HostValidator class to validate hostnames and IP
addresses entered into a QLineEdit. We use this class to validate the
server and proxy host.

This also adds the existing PortValidator class to the server and
proxy port widgets.

fixes electroncash#2743

This is a backport of electroncash#2744

Run the GUI, test the server host, server port, proxy host and proxy port fields. Invalid hostnames or ports should cause the widget to have a red border.