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.

interface/wx/grid.h

@@ -1019,7 +1019,7 @@ protected:
 
     @since 2.9.1
  */
-class wxGridHeaderCellRenderer
+class wxGridHeaderLabelsRenderer
 {
 public:
     /**
@@ -1063,7 +1063,7 @@ public:
 /**
     Base class for row headers renderer.
 
-    This is the same as wxGridHeaderCellRenderer currently but we still use a
+    This is the same as wxGridHeaderLabelsRenderer currently but we still use a
     separate class for it to distinguish it from wxGridColumnHeaderRenderer
     and wxGridCornerHeaderRenderer.
 
@@ -1073,14 +1073,14 @@ public:
 
     @since 2.9.1
  */
-class wxGridRowHeaderRenderer : public wxGridHeaderCellRenderer
+class wxGridRowHeaderRenderer : public wxGridHeaderLabelsRenderer
 {
 };
 
 /**
     Base class for column headers renderer.
 
-    This is the same as wxGridHeaderCellRenderer currently but we still use a
+    This is the same as wxGridHeaderLabelsRenderer currently but we still use a
     separate class for it to distinguish it from wxGridRowHeaderRenderer and
     wxGridCornerHeaderRenderer.
 
@@ -1090,14 +1090,14 @@ class wxGridRowHeaderRenderer : public wxGridHeaderCellRenderer
 
     @since 2.9.1
  */
-class wxGridColumnHeaderRenderer : public wxGridHeaderCellRenderer
+class wxGridColumnHeaderRenderer : public wxGridHeaderLabelsRenderer
 {
 };
 
 /**
     Base class for corner header renderer.
 
-    This is the same as wxGridHeaderCellRenderer currently but we still use a
+    This is the same as wxGridHeaderLabelsRenderer currently but we still use a
     separate class for it to distinguish it from wxGridRowHeaderRenderer and
     wxGridColumnHeaderRenderer.
 
@@ -1107,7 +1107,7 @@ class wxGridColumnHeaderRenderer : public wxGridHeaderCellRenderer
 
     @since 2.9.1
  */
-class wxGridCornerHeaderRenderer : public wxGridHeaderCellRenderer
+class wxGridCornerHeaderRenderer : public wxGridHeaderLabelsRenderer
 {
 };