Add support for Thunderbird 78

Support for legacy add-ons has been dropped with Thunderbird 78 (https://developer.thunderbird.net/add-ons/updating/tb78). Therefore, XNote++ would require a major rewrite to work on Thunderbird 78 - not sure whether all of the required functionality is already available via MailExtension API. Therefore, one would probably also have to work with so called "Experiments" to get access to functionality not available via MailExtension API - see earlier link for details.