reader config dialog: properly refresh config pane

This serves as a good example for the way refreshes are done:
setDirty("all", function() ... end)
* the "all" will have all widgets on screen repainted.
  In this case that is needed because the config pane has
  different sizes, covering different parts of underlying
  widgets. So they need to be repainted every time.
* the function will return the area to refresh and is evaluated
  after painting. In this example, we take the area that is covered
  by the config pane before switching it (if present at all), and
  hand it to the refresh area function as an upvalue.
  When the function is called later after painting, it will
  use that saved area and combine it with the area that is covered
  then by the widget. That way, parts that are covered no more are
  included in the refresh area, too.