Deprecation/etrigan/warning

[simonmicro]

This PR adds a warning about pending removal of Etrigan support.

Why?

• Because Etrigan was not updated anymore since years - it works, but there is nobody to fix it if it breaks, so let's remove it now before it is too late.
• The Etrigan GUI is just some tkinter window. There is no real reason, why to limit it to Etrigan-only existence!
• Dropping GUI support will remove some boilerplate code from the client
• Dropping GUI code will remove MUCH code from the server + some libraries
• It gives us/me the ability to focus on developing a new web-based GUI, while the old is being stripped out at the same moment

[simonmicro]

whoops, wrong repo