The built-in OAuth2 service for Google and Outlook had only build-time
options to specify the client ID and secret, which is quite limiting
and unnecessarily complicated. The GSettings keys make it much simpler
to override the build-time values.

See https://developer.gnome.org/hig/stable/typography.html

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.

See https://developer.gnome.org/hig/stable/typography.html

It's a GNetworkMonitor implementation, a proxy for a real instance,
which allows to change in runtime which implementation of the GNetworkMonitor
will be used, together with a special option to report an always-online
state. This instance can be used by any part, thus the changes in it
influence not only Mail, but also Contacts, Calendars, Memo Lists and
Task Lists. It wasn't easily possible with the raw GNetworkMonitor.

The added GSettings option is:
   org.gnome.evolution-data-server network-monitor-gio-name
and the ENetworkMonitor uses it transparently.

This allows show of signers photo, when the public key has any and
the gpg/2 provides it. This behavior is enabled by default, but
it can be disabled by:
   $ gsettings set org.gnome.evolution-data-server camel-cipher-load-photos false

The size of public struct _CamelCipherCertInfo changed, thus a soname version
bump should be done, but as it had been changed recently and no release of
the evolution-data-server had been made meanwhile, then it was not changed.