Replace boost::shared_ptr<TextClass> with a global cache of sorts of the TextClass...

Replace boost::shared_ptr<TextClass> with a global cache of sorts of the TextClass's used by Buffers---or, more strictly, constructed by BufferParams::makeTextClass(). The action is in TextClass.{h,cpp}.

I've left the typedef in TextClassPtr.h. At the moment, it's kind of silly. But I've left it mostly because it helps to identify where the TextClass's stored in the TextClassBundle are used, and maybe it'd be worth having some sort of strong typedef like the one for BaseClassIndex here.

I need to check whether the textClass_ member of InsetCollapsable is needed now. I think not.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@23232 a592a061-630c-0410-9148-cb99ea01b6c8