composer-autosave: Miscellaneous cleanups.

(cherry picked from commit 2a69370d81d5adf74450ef19b36d9e7e400ff4d5)

Conflicts:
	modules/composer-autosave/e-composer-autosave.c
Bug 712668 - Disable message recovery after initial scan

This prevents Evolution from prompting to restore what it thinks are
orphaned messages during an Evolution session.  This is triggered by
a shell window emitting another "map-event" signal for some reason.

Some GNOME Shell users have reported seeing this.

(cherry picked from commit 66923efa0ce1cb1d00440010b461ad463a200186)