This part adds an "Import as Event" button to the meeting request
window when the selected calendar claims it can add only meetings
organized by the calendar owner, which will remove the organizer
and all the attendees before the import.
Related to https://gitlab.gnome.org/GNOME/evolution-ews/-/issues/295
All the previews (in Mail, Contacts, Memos and Tasks) stop using
deprecated WebKitGTK+ DOM API through a WebExtension (and its D-Bus
API) and start using the JavaScriptCore API (and JavaScript as such).
This allows to lightweight the WebExtension for the EWebView
significantly, including drop of the D-Bus API of it, fully relying
on the Inter-Process-Communication of WebKitGTK+ itself.
