Give MailSession a permanent home.
Global variables in shared libraries are a bad idea. EMailBackend now owns the MailSession instance, which is actually now EMailSession. Move the blocking utility functions in mail-tools.c to e-mail-session.c and add asynchronous variants. Same approach as Camel. Replace EMailReader.get_shell_backend() with EMailReader.get_backend(), which returns an EMailBackend. Easier access to the EMailSession.
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