supercollider: add Emacs support (aka SCEL)

The current expression for SuperCollider in the repository builds and installs a version of the application which uses a built-in Qt-based IDE, but disables built-in support for using Emacs as an IDE.  This new expression enables that feature and also builds the built-in Qt IDE by default.  I used the current SuperCollider default expression as a staring point, and then modified the proper cmake flag and added the necessary dependencies.