A simple Evolution run and move between all views means creation of
more than 100 GSettings objects, with only a bit more than 10 schemas.
Reusing the objects should have a positive impact on a performance too.
EShellSettings predates GSettings and is no longer necessary.
GSettings allows binding GObject properties to GSettings keys,
with optional mapping functions. That fulfills the purpose of
EShellSettings.
Collect all the "config" extensions from the "addressbook", "calendar",
and "mail" modules into one place. These extensions typically just bind
GObject properties of extensible classes to app-specific GSettings keys.
