emacs: Separate package license code.

Move list/info interface code from "guix-license.el" to
"guix-ui-license.el".

* emacs/guix-license.el (guix-license-get-entries, guix-license-get-display)
(guix-license-insert-packages-button, guix-license-insert-comment)
(guix-licenses): Move to...
* emacs/guix-ui-license.el: ... here.  New file.
* emacs.am (ELFILES): Add it.