Git context menu (#24844)

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

Cargo.lock

@@ -5370,6 +5370,7 @@ dependencies = [
  "serde_derive",
  "serde_json",
  "settings",
+ "strum",
  "theme",
  "time",
  "ui",