imap-smtp: Refactor the encryption combo-box code
imap-smtp: Refactor the encryption combo-box code

... to avoid duplicating translatable strings. This way an inadvertent
mistake won't lead to spurious strings, and so on.

Add a comment for translators mentioning the similarity of the strings
with evolution.

Fixes: https://bugzilla.gnome.org/695559

halfline/gnome-online-accounts,ritzk/gnome-online-accounts,halfline/gnome-online-accounts,ritzk/gnome-online-accounts,felipeborges/gnome-online-accounts,felipeborges/gnome-online-accounts,dnohales/gnome-online-accounts,ritzk/gnome-online-accounts,Distrotech/gnome-online-accounts,dnohales/gnome-online-accounts,Distrotech/gnome-online-accounts,halfline/gnome-online-accounts,dnohales/gnome-online-accounts,Distrotech/gnome-online-accounts,felipeborges/gnome-online-accounts