Make file organisation in XCode/VS match dirs

CMake allows you to specify a separate hierarchy of folders/files that
appear in an XCode or visual studio project. Previously we had
everything in the various src directories at the root of each module.
Now we have a src, test, and headers directory for each module.
Move text package functions into DocFormats

The Word and HTML test code that currently lives in dfutil (but not for
much longer) uses plain text representations of "packages", which are
collections of files e.g. like you might find in a .docx file or a
directory with a HTML file and some images. With the new directory
organisation where we keep tests in the DocFormats tree itself (so as to
not require access to public APIs from an external program), we need
this functionality in the library before we can move the test functions
in.

Currently, the actual test functions which use these plain text
representations are still in dfutil; however they will soon be moved to
the tests directory within the the html and ooxml directories.
Provision for plain and data test types
Remove Xcode projects

These are no longer needed, since for a long time now we've been
generating Xcode projects from CMake. I've been maintaining these
project files up until now because because I still haven't got iOS & OS
X building working simultaneously for the purposes of UX Write, but they
no longer belong in the main repository (too much effort to keep them up
to date with the CMake files) and I'll sort out iOS later.