emacs: Add "Build" button to Package Info.

* emacs/guix-main.scm: Use (guix scripts) module for 'build-package'
procedure.
* emacs/guix-base.el (guix-build-package): New procedure.
* emacs/guix-ui-package.el (guix-package-info-insert-build-button): New
procedure.
(guix-package-info-button-functions): New variable.
(guix-package-info-insert-misc): Insert buttons using it.