the boost::signals into the relevant Impl classes and creating new,
publicly-accessible connect() methods.
* Const-ify all publicly-accessible methods, reflecting the fact that each
class contains only a const pimpl_ member variable.
git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@4687 a592a061-630c-0410-9148-cb99ea01b6c8
