Radiobuttons

Hello,

I've implemented some kind of radiobuttons; also moved some code out of lTerm_widget.ml (into lTerm_widget_callbacks.ml and some internal modules in src/widget_impl) to prevent it from growing uncontrollably. API for pre-existing stuff didn't change.

I also added .merlin configuration file for merlin tool -- is it ok to keep it in the repository?

P.S.
- no default resources for buttons yet
- I've failed to implement minimal example of documentation bug to make report to oasis. I've made a file which loses one doc comment when parsed with camlp4o, checked that oasis build process really calls ocamldoc with -syntax camlp4o for this file -- but somehow generated documentation does contain lost comment. I'm utterly confused. Will keep looking.