Rename a class to restore backward compatibility

This solves backward incompatibility problem introduced in 2d8bbbe3c9.
The name of base class for grid header renderers hierarchy is restored
to be wxGridHeaderLabelsRenderer. This introduces another problem: now
wxGridCornerHeaderRenderer is derived from wxGridHeaderLabelsRenderer,
not vice versa, as it was before. But it is considered less disruptive
change, compared to base class rename.

include/wx/generic/grid.h

@@ -326,10 +326,10 @@ protected:
 // ----------------------------------------------------------------------------
 
 // Base class for header cells renderers.
-class WXDLLIMPEXP_CORE wxGridHeaderCellRenderer
+class WXDLLIMPEXP_CORE wxGridHeaderLabelsRenderer
 {
 public:
-    virtual ~wxGridHeaderCellRenderer() {}
+    virtual ~wxGridHeaderLabelsRenderer() {}
 
     // Draw the border around cell window.
     virtual void DrawBorder(const wxGrid& grid,
@@ -347,21 +347,21 @@ public:
 };
 
 // Currently the row/column/corner renders don't need any methods other than
-// those already in wxGridHeaderCellRenderer but still define separate classes
+// those already in wxGridHeaderLabelsRenderer but still define separate classes
 // for them for future extensions and also for better type safety (i.e. to
 // avoid inadvertently using a column header renderer for the row headers)
 class WXDLLIMPEXP_CORE wxGridRowHeaderRenderer
-    : public wxGridHeaderCellRenderer
+    : public wxGridHeaderLabelsRenderer
 {
 };
 
 class WXDLLIMPEXP_CORE wxGridColumnHeaderRenderer
-    : public wxGridHeaderCellRenderer
+    : public wxGridHeaderLabelsRenderer
 {
 };
 
 class WXDLLIMPEXP_CORE wxGridCornerHeaderRenderer
-    : public wxGridHeaderCellRenderer
+    : public wxGridHeaderLabelsRenderer
 {
 };