The only two parts using it were GDBus objects for address book and
calendar view, which could be easily transformed to use gdbus-codegen,
the same as the rest of the GDBus objects defined in src/private/.

Related to https://gitlab.gnome.org/GNOME/evolution/issues/196

Closes https://bugzilla.gnome.org/show_bug.cgi?id=228772

This object can be used by applications which want to implement
their own notifications for scheduled reminders without a need
to implement the core functionality.

A follow-up change, with more aggressive fix, which doesn't start
factory subprocesses but runs backends in the main factory process.

It looks like the IMAP accepts an @outlook.com address, while
SMTP accepts only the primary address of the Microsoft account,
which not necessarily is the same @outlook.com address.

This change allows easier extending of built-in OAuth2 authentications
with minimal code "duplication".

A CMake option ENABLE_GOOGLE_AUTH had been renamed to ENABLE_OAUTH2
to reflect this extensibility as well.

This covers bug 784208, gtk-doc generated warnings like this one:
Warning: multiple "IDs" for constraint linkend: api-index-3.2.