Add wxListCtrl::SetHeaderAttr()

This method can be used to change the list view header appearance.

Add the method declaration, documentation, show it in the sample and implement
it for wxMSW (only, for now).

include/wx/listbase.h

@@ -411,6 +411,9 @@ public:
     void SetAlternateRowColour(const wxColour& colour);
     wxColour GetAlternateRowColour() const { return m_alternateRowColour.GetBackgroundColour(); }
 
+    // Header attributes support: only implemented in wxMSW currently.
+    virtual bool SetHeaderAttr(const wxItemAttr& WXUNUSED(attr)) { return false; }
+
     // Checkboxes support: only implemented in wxMSW currently.
     virtual bool HasCheckboxes() const { return false; }
     virtual bool EnableCheckboxes(bool WXUNUSED(enable) = true) { return false; }