Sets the formPanel private field
Checks that layers are instance of FLyrVect before returning it

If the group (FLayers) and the layer (FLyrVect) have the same name
a Cast Exception is raised because is getting the name of the group
before the name of the layer.

BTW a couple of utility methods are added to make the code a bit
more readable
Adds tests for TOCLayerManager class

NavTable should be updated, as it delegates on some code pushed to it