Also following the guidelines at https://gitlab.gnome.org/Teams/Design/settings-mockups/-/raw/master/remote-desktop/remote-desktop.png
Fixes #1413 (closed)
After discussions with @aday and @jadahl, we decided to drop !1135 (closed) as a whole panel and instead go with a dialog within the Sharing panel.
This MR is incomplete and is being posted for design/UI discussions.