Emacs: Denote version 2.3.0

This release brings a host of user-facing refinements to an already stable base, as well as some impressive new features. There is a lot to cover, so take your time reading these notes.

Special thanks to Jean-Philippe Gagné Guay for the numerous refinements to parts of the code base. Some of these are not directly visible to users, but are critical regardless. In the interest of brevity, I will not be covering the most technical parts here. I mention Jean-Philippe’s contributions at the outset for this reason. Though the Git commit log is there for interested parties to study things further.