Add an option to not display a gateway on the gateways dashboard widget

Quite often I have gateways that I do not care about, e.g. to a test network that is not always turned on, or a gateway with monitoring disabled (and in that case the dashboard widget always shows it green and with 0ms latency and 0% loss). Rather than having those gateways always showing up on the dashboard widget, I thought it would be handy to be able to not display them on the dashboard.
Note: They do all still display in Status->Gateways. IMHO that list should stay as a complete list.
So, here is the code, if you think this is a useful option.