[evolution] Plea to include a migration code for 3.28.0

[Milan Crha <mcrha redhat com>]

        Hello,
I'd like to ask for an approval to commit a code change into Evolution
for 3.28.0, which adds a migration code for changes in bug [792076].

There had been requested to change the default value for one GSettings
key and it had been committed for 3.27.90. I expected that existing
users won't be affected by the default value change, but in time of the
commit I noticed it's not true for users whom never touched the value
of the option, because GSettings/DConf uses the default value from the
scheme and when it flips in the scheme, then it flips also for such
existing installations.

This behaviour is bad and unexpected by the users, but I didn't know
what to do with it. Only after I semi-announced such behaviour for
3.28.0 on the evolution-list, I noticed there's a GSettings API to
cover it. I attached the patch which adds necessary migration to the
bug [792076], but I'd really prefer to commit it for 3.28.0, thus the
affected existing users won't be disappointed. It's a minor change
code-wise speaking, but an important change for the users.
        Thanks and bye,
        Milan

[792076] https://bugzilla.gnome.org/show_bug.cgi?id=792076