Adds the non-entry specific right click menu to the panel, and the
features contained therin:
* Stage all
* Discard Tracked Changes
* Trash Untracked Files
Also changes the naming from "Changes"/"New" to better match Git's
terminology (though not convinced on this, it was awkward to describe
"Discard Changes" without a way to distinguish between the changes and
the files containing them).
Release Notes:
- N/A