There are lot of places where constants are used in the KiCad UI
as "magic numbers". The most common one is "5", used in many
wxFormBuilder and manual UI constructions as the margin.
This commit provides a place for all UI to look up shared
constants and other functions, to help create a consistent UI using
functions that provide meaning and intent to these magic numbers.
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