Use a `settings` window to let user make customized settings

[yuankunzhang]

We could use a settings popup window to let user set preferences like:

• Listen on what address:port
• Dart/light theme
• Path to custom certificate
• ...

[emanuele-em]

@yuankunzhang after last update we introduced an initial setting popup where you can select the address and the port, I think that dark/light theme toggle is a standard in a window corner and so i prefer to keep it there, regarding the custom path I think that is a very good idea, if you want I can assign you this issue:

• add a browse file button in the initial menu
• check that the file is a real certificate
• (probably is better to save that certificate in local machine certificate store and check if it is valid on each startup)