Task: Generalize File::XLIFF to work on zipped XLIFF

The files Lionbridge uses in their XLIFF editor are actually zipped XLIFF (with a .xlz extension) and include a "skeleton" file that seems to have some kind of information about placeables.

It would be nice to have a way of dealing with those for batch manipulation (global find-and-replace, etc.).

XLIFF

So I started a File::XLIFF module yesterday. XLIFF [spec] is an interesting format. Like many XML formats, it's overengineered to the point that I suspect nobody will ever use it to its fullest extent. It maps onto the much simpler TTX format only with a lot of folding, spindling, and mutilation.

The basic Xlat::File model of a file as a simple set of segments may turn out to be oversimplified when it comes to XLIFF. As the most obvious example, a single XLIFF file can contain multiple sections, each of which refers to content from a separate file, and thus each of which has its own header and its own body.

Under the assumption that an XLIFF file will usually correspond to a single source file, I'm going to define a "default section" (that being the first in the file) that will be the target for the API against the file object; using XLIFF-specific functions, I'll expose a way to get a list of sections and create a separate file object pointing to a section that's numbered 2 or above.

Each of the File:: modules should probably have an Xlat::File superclass. I don't want to introduce needless dependencies, though; perhaps I can test for installation of Xlat::File before superclassing? Or maybe this is a plate of beans.