This is in preparation for making this widget optionally read-only.
Major changes:
* Construct panel in code, not with wxFormBuilder. This make's it
easier to conditionally construct elements that won't be used
in a read-only mode (e.g. the buttons).
* Use a generic "button row panel" widget for the buttons, as the
sizing and layout logic is reusable in nearly all dialogs, and
it's simplifies layout in the higher-level dialog widget. This
widget is one example of many possible "reuable widgets".
John Beard