general: merge libnautilus-private to src

And fix make distcheck.

Although libnautilus-private seem self contained, it was actually
depending on the files on src/ for dnd.
Not only that, but files in libnautilus-private also were depending on
dnd files, which you can guess it's wrong.

Before the desktop split, this was working because the files were
distributed, but now was a problem since we reestructured the code, and
now nautilus being a library make distcheck stop working.

First solution was try to fix this inter dependency of files, but at
some point I realized that there was no real point on splitting some of
those files, because for example, is perfectly fine for dnd to need to
access the window functions, and it's perfectly fine for the widgets
in the private library to need to access to all dnd functions.

So seems to me the private library of nautilus is somehow an artificial
split, which provides more problems than solutions.

We needed libnautilus-private to have a private library that we c...