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).

samples/listctrl/listtest.h

@@ -132,6 +132,7 @@ protected:
     void OnSetFgColour(wxCommandEvent& event);
     void OnSetBgColour(wxCommandEvent& event);
     void OnSetRowLines(wxCommandEvent& event);
+    void OnCustomHeaderAttr(wxCommandEvent& event);
     void OnToggleMultiSel(wxCommandEvent& event);
     void OnShowColInfo(wxCommandEvent& event);
     void OnShowSelInfo(wxCommandEvent& event);
@@ -221,6 +222,7 @@ enum
     LIST_SET_FG_COL,
     LIST_SET_BG_COL,
     LIST_ROW_LINES,
+    LIST_CUSTOM_HEADER_ATTR,
     LIST_TOGGLE_MULTI_SEL,
     LIST_TOGGLE_HEADER,
     LIST_TOGGLE_BELL,