Dictionary now uses button

Dictionary now uses button

* net/lisp/dictionary-link.el: Removed now obsolete file
* net/lisp/dictionary.el: Use insert-button and make-button
* net/lisp/dictionary.el (dictionary-mode-map): Now defined using defvar

I had to add a conversion function as parameter for the button 'action
as I need to be able to pass nil data to my function. This is not
possible with the regular button 'action function and the 'button-data
value.

The functionality of searching a link in all dictionaries has been
removed for now. It might appear again once I have an idea how to
implement it.