Avalonia App: add missing features from WinForms App

[jochenwezel]

Check tickets if already implemented at Avalonia app

• Prevent mis-use: Ensure remote server is a Guacamole server #16
• Name of remoted server in form title + menu bar #15
• Menu entry: Settings + Connections #10
• Multiple configured Guacamole connection profiles
• Mainform's title bar and menu bar in colors defined by current connection profile
  • working on all main target platforms of the Avalonia app
    • Win
    • Mac
    • Linux