Minimize to tray when the taskbar-icon is clicked

[D4koon]

• WhatsApp-window is open and has focus:
• Clicking the taskbar-icon of whatsapp minimizes it to the taskbar instead of tray
• This should be configurable by a command-line-switch.
• The default setting should be as it is currently i think.

This is a followup to #30

[ghost]

yes please