Add support for arbitrary window labels in wxStaticBox to wxMSW

Just reparent the label window and position it accordingly and, also,
avoid painting over it in MSW-specific code.

include/wx/msw/statbox.h

@@ -27,6 +27,16 @@ public:
         Create(parent, id, label, pos, size, style, name);
     }
 
+    wxStaticBox(wxWindow* parent, wxWindowID id,
+                wxWindow* label,
+                const wxPoint& pos = wxDefaultPosition,
+                const wxSize& size = wxDefaultSize,
+                long style = 0,
+                const wxString &name = wxStaticBoxNameStr)
+    {
+        Create(parent, id, label, pos, size, style, name);
+    }
+
     bool Create(wxWindow *parent, wxWindowID id,
                 const wxString& label,
                 const wxPoint& pos = wxDefaultPosition,
@@ -34,6 +44,13 @@ public:
                 long style = 0,
                 const wxString& name = wxStaticBoxNameStr);
 
+    bool Create(wxWindow *parent, wxWindowID id,
+                wxWindow* label,
+                const wxPoint& pos = wxDefaultPosition,
+                const wxSize& size = wxDefaultSize,
+                long style = 0,
+                const wxString& name = wxStaticBoxNameStr);
+
     /// Implementation only
     virtual void GetBordersForSizer(int *borderTop, int *borderOther) const wxOVERRIDE;
 
@@ -66,5 +83,8 @@ protected:
     wxDECLARE_DYNAMIC_CLASS_NO_COPY(wxStaticBox);
 };
 
+// Indicate that we have the ctor overload taking wxWindow as label.
+#define wxHAS_WINDOW_LABEL_IN_STATIC_BOX
+
 #endif // _WX_MSW_STATBOX_H_