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.
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