MSWrite plugin cleanup and reformatting

I've completely cleaned up the MSWrite plugin source code and consistently
formatted it to the AbiWord coding style. Due to lots of renamings and
reorderings, it is much easier to understand now and coming improvements and
fixes will be much easier to perform. The plugin itself, of course, has been
tested to be still as stable as it was before.

If nobody objects I'll commit shortly.

Ingo