All this so EMFolderTree can submit EActivity instances for async ops.
You can obtain an EMailSession from an EMailBackend, but not vice versa.
Creates lots of pretty ripples in the mail code, but ultimately reduces
complexity. So it's a code cleanup of sorts.
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.
Convert the "startup-wizard" EPlugin to an EExtension, and fix up the
importing UI a bit (but it still needs a lot more love). Importing
progress is now shown directly in the GtkAssistant window.
Define a new EConfigItem type (E_CONFIG_PAGE_PROGRESS) for creating
progress pages in a GtkAssistant.
Also, change EMAccountEditor semantics slightly: you now have to call
e_config_create_window() manually after creating a new EMAccountEditor
instance. This allows extra EConfigItems (specifications for the window
content) to be added manually before the window is created.
