Use custom draw in wxMSW wxHeaderCtrl to support colours

Add a helper wxMSWImpl::CustomDraw class which will be reused in the other
places too and, for now, use it just to implement support for custom colours
in wxHeaderCtrl.

Notice that the control took care of the custom font on its anyhow and that
background colour is ignored when themes are enabled, so the net effect of
this change is that now changing the header foreground colour works, while
it was ignored before.

include/wx/msw/headerctrl.h

@@ -12,6 +12,8 @@
 
 class WXDLLIMPEXP_FWD_CORE wxImageList;
 
+class wxMSWHeaderCtrlCustomDraw;
+
 // ----------------------------------------------------------------------------
 // wxHeaderCtrl
 // ----------------------------------------------------------------------------
@@ -45,7 +47,11 @@ public:
 
     virtual ~wxHeaderCtrl();
 
-    
+    // Override to implement colours support via custom drawing.
+    virtual bool SetBackgroundColour(const wxColour& colour) wxOVERRIDE;
+    virtual bool SetForegroundColour(const wxColour& colour) wxOVERRIDE;
+    virtual bool SetFont(const wxFont& font) wxOVERRIDE;
+
 protected:
     // override wxWindow methods which must be implemented by a new control
     virtual wxSize DoGetBestSize() const;
@@ -98,6 +104,10 @@ private:
     // mouse button
     wxEventType GetClickEventType(bool dblclk, int button);
 
+    // allocate m_customDraw if we need it or free it if it no longer is,
+    // return the pointer which can be used to update it if it's non-null
+    wxMSWHeaderCtrlCustomDraw* GetCustomDraw();
+
 
     // the number of columns in the control, including the hidden ones (not
     // taken into account by the native control, see comment in DoGetCount())
@@ -129,6 +139,10 @@ private:
     // actual column we are dragging or -1 if not dragging anything
     int m_colBeingDragged;
 
+    // the custom draw helper: initially NULL, created on demand, use
+    // GetCustomDraw() to do it
+    wxMSWHeaderCtrlCustomDraw *m_customDraw;
+
     wxDECLARE_NO_COPY_CLASS(wxHeaderCtrl);
 };