Fix wxMSW wxSpinCtrl appearance: show arrows inside the control

As recommended in the "Spin Controls" MSDN documentation (see
https://msdn.microsoft.com/en-us/library/windows/desktop/dn742439.aspx), put
the spin control inside the associated "buddy" edit control and not near it.

Closes #12297.

Closes https://github.com/wxWidgets/wxWidgets/pull/410

include/wx/msw/spinctrl.h

@@ -159,6 +159,9 @@ private:
     // hexadecimal prefix, ...) in it.
     void UpdateBuddyStyle();
 
+    // Determine the (horizontal) pixel overlap between the spin button
+    // (up-down control) and the text control (buddy window).
+    int GetOverlap() const;
 
     wxDECLARE_DYNAMIC_CLASS(wxSpinCtrl);
     wxDECLARE_EVENT_TABLE();