[evolution] String freeze break plea for gnome-3-26

[Milan Crha <mcrha redhat com>]

        Hello,
during 3.25 development cycle I made a change for [1], where I marked
Fwd and Re attributions for translation, as requested by the reporter.
What I didn't realize at that moment was that people in
internationalized environments might have problem with it (not talking
that the "Re" is part of RFC, which I missed completely). It stroke
shortly after those users begun to use 3.26 in locale which had
translated those strings. It resulted in bug [2].

I just committed a fix for it for 3.27.1 [3], but I'd like to add the
change also for 3.26.2. The original change contains also UI changes,
because I added a checkbox for it into the Composer Preferences, but
I'd like to simplify the change for 3.26 and include only translatable
strings in GSettings [4]. The default value for the option is not to
use translated Fwd/Re, which will fix it for majority of the users and
those whom prefer the translated strings can change the values of the
option manually or wait for 3.28.0.

I believe that regular users access GSettings/DConf only when told to
do it, thus these two strings [4] not being translated should not cause
any significant harm, especially in compare of the issue itself.

        Thanks and bye,
        Milan

[1] https://bugzilla.gnome.org/show_bug.cgi?id=783968
    Mark Fwd and Re Subject attribution for translation

[2] https://bugzilla.gnome.org/show_bug.cgi?id=788916
    Localized "Fwd:"/"Re:" can't be disabled

[3] https://git.gnome.org/browse/evolution/commit/?id=c5ca2d356f

[4] <_summary>Use localized “Fwd”/“Re” in message Subject</_summary>
    <_description>When set to true, uses localized “Fwd”/“Re” in
    message Subject on reply and forward as provided by current locale
    translation, otherwise uses unlocalized version.</_description>