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.
Similar to GObject::notify, the GSettings::changed can be emitted
even if a key didn't change. It's up to the user (aka evolution)
to test for real changes, thus let's do it. It may have certain
performance positive impact too.
These libraries are bound for E-D-S so they live at the lowest layer of
Evolution for now -- even libeutil can link to them (but please don't).
This is the first step toward moving mail handing to a D-Bus service.
