- Added "layers" class, that implements the layer management system;
 - there is one single dictionary that stores layers (value) and their names (key)
 - All layer names are stored inside this class, as internationalized constants: cannot really allow them to come from anywhere else
 - we only remove layers that were introduced by us (don't touch the user's layers!)
 - if a layer is not present in the data directory (the user removed it?) we throw an error (can not really allow this to happen!);

- created a method on the "page" abstract class, that allows to define each layers we show at each page; we ask the manager to show the requested layers and hide everything else;
- when loading layers, a zoom to full is always called, to adjust the extents
- passed a reference to the wizard to all pages, through the constructor
- amended the ID's of the pages, to have a numerical sequence
- amended text on the first page of the wizard