Fix client coordinates mapping for wxSpinCtrl in wxMSW

Add MSWDoClientToScreen() and MSWDoScreenToClient() helpers and use them
with the correct HWND in DoClientToScreen() and DoScreenToClient()
overridden in wxSpinCtrl, i.e. the HWND of the "buddy" text control and
not the spin button, which is the main HWND of this control.

This notably fixes wxSpinCtrl::GetScreenPosition() which returned the
position of the spin button.

Closes https://github.com/wxWidgets/wxWidgets/pull/1454

Closes #18455.

src/msw/spinctrl.cpp

@@ -846,4 +846,14 @@ void wxSpinCtrl::DoGetPosition(int *x, int *y) const
     *x = wxMin(xBuddy, xText);
 }
 
+void wxSpinCtrl::DoScreenToClient(int *x, int *y) const
+{
+    wxWindow::MSWDoScreenToClient(GetBuddyHwnd(), x, y);
+}
+
+void wxSpinCtrl::DoClientToScreen(int *x, int *y) const
+{
+    wxWindow::MSWDoClientToScreen(GetBuddyHwnd(), x, y);
+}
+
 #endif // wxUSE_SPINCTRL