Refactor wxGridSelection to store selection as blocks only

Store all types of selection with an array of blocks instead of arrays of
cells, blocks, rows and columns.

It (hopefully) simplifies the code and allows us to implement editing of
the last selection block much easier.

interface/wx/grid.h

@@ -1904,7 +1904,7 @@ public:
     wxGridBlockCoords Canonicalize() const;
 
     /**
-        Whether the blocks intersects.
+        Whether the blocks intersect.
 
         @return
             @true, if the block intersects with the other, @false, otherwise.