App.config reading and writing

    // Prepared for you by Predrag Gjuro Kladarić, 2023-07-08

    // first, use "Manage NuGet Packages" to install "System.Configuration.ConfigurationManager"

    // to add App.config file to project root folder, use
    //          Add ->
    //          New Item -> 
    //          Installed ->
    //          C# Items ->
    //          Application Configuration File
    // 
    // if you add it, this config file will be copied to runtime folder (bin/Debug/net7.0) on first run
    // if you don't, SaveAppSettings() will create one (but on runtime folder)
    //
    // beware, SaveAppSettings() creates/updates App.config file on the runtime folder