Feature: List of already assigned fixed public ports

[Flachdachs]

When you create a stored tunnel configuration that should have a fixed public port - because arbitrary ports are cumbersome when you want to use the tunnel with a 3rd-party tool that has its own connection configurations and the port always changes - you have to write down elsewhere which ports are already assigned. It would be nice to be able to see such a list when configuring the port, e.g. having a button next to the field that opens a dialog.