support for multirows in tables:

support for multirows in tables:
- fileformat change (lyx2lyx code follows the next days)
- the patch is a joint work of Edwin and me

The patch is huge but the major parts work already. I put it in now as the minor known issues can be solved step by step the next days. I think this is the best solution because collaborating with such large patches already lead to copy/paste bugs.

The known issues are listed here:
http://wiki.lyx.org/Devel/Multirow

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@33414 a592a061-630c-0410-9148-cb99ea01b6c8