Draft: Introduce Remote Desktop panel

Fixes #1413 (closed)

WIP!

It currently has some rough widgets for setting the server certificate and key. As of now, it can't generate certificates. I will follow up using Gcr with its GcrUi widgets for selecting certificates and all.

Other than that, the panel is already plugged in to the backend and it works on my local tests.

Testing

You can generate certificates with

openssl genrsa -out tls.key 4096
openssl req -new -key tls.key -out tls.csr
openssl x509 -req -days 730 -signkey tls.key -in tls.csr -out tls.crt

And select the generated files (tls.crt and tls.key) in the UI.

The steps above were taken from https://gitlab.gnome.org/-/snippets/1778