As discussed recently on the users mailing list [1], there's a strong
precedent among GTK+ applications for F9 to toggle sidebar visibility.
Examples cited were Nautilus, Evince, Totem, Rhythmbox, File-Roller
and gThumb.
Change the Send/Receive accelerator from F9 to F12, and designate F9
as the accelerator for View->Layout->Show Side Bar.
[1] https://mail.gnome.org/archives/evolution-list/2013-May/msg00194.html
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