So, I was storing a single "sd" pointer (class private data)
but using it in two different places, ie. as if it were two
different classes. Crashes happened, complaints were heard.
See T3714.
This touches Edje and also Elementary where part names are used.
This commit blew up in size since now all content part APIs
(get, set, unset) require to use Efl.Part instead.
This is a big refactoring commit, but no logic should
have been changed. Fingers crossed.