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.
This will replace the headers API in EMailFormatter. Need a more
permanent place for headers since EMailFormatter is too disposable.
Also add an ESettingsMailPartHeaders class, which binds the new property
to the "show-headers" setting with a suitable mapping function to filter
out disabled header names.
Milan Crha