Navigate the frames of a stacktrace.

While a trace is active, allow navigation up and down the call stack with a `NavigationView` that attaches to the bottom of the workspace.

- [x] Create a panel view and attach it to the bottom of the workspace.
- [x] Show the name of the active stacktrace. If there is no active stacktrace, hide the view, but don't destroy it.
- [x] Navigate back to the `StacktraceView`.
- [x] Allow the stacktrace to be deactivated.
  - [x] Remove the activation controls from the `StacktraceView`, it makes more sense here.
- [x] Show the current frame and its position within the trace.
- [x] Navigate to the next and previous frames.
- [x] Notices if you manually navigate to a different frame, within the same editor or a new one.
- [x] Distinguish visually between being right on a frame and having moved off of one, but not onto another yet.
- [x] Navigate to the last active frame.