Right now the OverviewMap plugin uses a regular QPainter with x and y coordinates and some custom calculations to display geodetic features (boundingrect, focusPoint, etc).

In Marble we have the GeoPainter class which can draw geodetic features for a given projection.

This task is about refactoring the OverviewMap plugin to make use of GeoPainter:

1. Create a custom ViewportParams instance initialized to match the size and projection of the overview map (EquiRectangular projection, radius mapRect.height / 2)
2. Draw the boundingRect and the focus point using a new instance of GeoPainter on top of the overviewmap plugin as a paintDevice and the custom viewportParams.

In the end the overview map should work and look the same as right now - but using GeoPainter.