Add Etesync

Adding EteSync support would be great. 😃

See also these feature requests:

• evolution-etesync#2
• https://github.com/etesync/etesync-dav/issues/5

Now that EDS support is here, what is needed to have this integrated into Gnome Online Accounts?
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Just the UI, yeah. The UI in Gnome Online Accounts is hard-coded to only allow certain resources, so need to send them a patch.