This simplifies the (already simple) SELECTION_AREA class. It is also
moved into KIGFX::PREVIEW and put in the common library, where it can be
reused by other GAL-aware tools (not just in Pcbnew) in future.
This introduces SIMPLE_OVERLAT_ITEM, which is an abstract EDA_ITEM,
designed for use as an overlay for assisting interactive tools.
The item is drawn only on the GP_OVERLAY layer, and sets the fill,
stroke and line properties before calling a virtual function to draw the
shape, which will depend on the implementaiton of the derived class.
The motivation here is to simplify and unify basic overlays used when
in interactive tools. It is not designed to be the base class of all
possible overlays - more complex ones would be more clearly represented
as their own derivative of EDA_ITEM.
Applications of this class can include: zoom/select rectangles, zone
polygon previews, geometric shape helpers, and so on.
John Beard