Generate IDE project files for Visual Studio 2010 and 2012.

Thanks to Mike Edenfield for the initial version of this patch; I've
polished it up a bit (in particular inventing a more overengineered
GUID generation strategy) but most of it is his.


git-svn-id: svn://svn.tartarus.org/sgt/putty@10112 cda61777-01e9-0310-a592-d414129be87e