Closes https://gitlab.gnome.org/GNOME/evolution/-/issues/1847
Move the plugins preferences into the preferences window, allows to have one less dialog to look-at to configure evolution. Closes https://gitlab.gnome.org/GNOME/evolution/-/merge_requests/84
Closes https://gitlab.gnome.org/GNOME/evolution/-/issues/1255
Closes https://gitlab.gnome.org/GNOME/evolution/merge_requests/23
Let's have it as it's common to be, which means top level src/ for sources, single data/ for data, and so on.