Configure "Allow connections from localhost only" checkbox

[jjspark]

Proposed feature
Similar to how Readonly API checkbox is configured with a config key in config.ini file, control the "Allow connections from localhost only" checkbox in the Settings dialog.

Proposed config key is AllowLocalhostOnly, and its possible values are yes/no, with default value of yes.

Motivation for the feature
I use IBC within a docker-compose environment, where I have IB Gateway with IBC running in one container, and my bot trader running in another container. I need IB gateway to accept connections from the other container in the docker network.

[rlktradewright]

It's not enough to just clear Allow connections from localhost only. The Trusted IP Address list also has to be set. This is work that I definitely won't have either the time or the inclination to do before 1 September (which is when I will be wrapping up the project. See Discussion #347 if you're not aware of this.

In any case, there is a better way to do this that does't involve any changes to IBC: this is to use an SSH tunnel to connect from your API application to the Gateway API port, which allows the connection to appear as if its coming from localhost.