Improve best size determination for wxSpinCtrl

Use the range and the base to determine the widest value string and use
it to calculate the best size.

include/wx/msw/spinctrl.h

@@ -168,6 +168,9 @@ private:
     // (up-down control) and the text control (buddy window).
     int GetOverlap() const;
 
+    // Calculate the best size for the number with the given number of digits.
+    wxSize GetBestSizeFromDigitsCount(int digitsCount) const;
+
     wxDECLARE_DYNAMIC_CLASS(wxSpinCtrl);
     wxDECLARE_EVENT_TABLE();
     wxDECLARE_NO_COPY_CLASS(wxSpinCtrl);

include/wx/spinctrl.h

@@ -143,6 +143,11 @@ namespace wxPrivate
 // string containing hexadecimal representation of the given number.
 extern wxString wxSpinCtrlFormatAsHex(long val, long maxVal);
 
+// The helper function to determine the best size for the given control.
+// We can't implement this function in the wxSpinCtrlBase because MSW implementation
+// of wxSpinCtrl is derived from wxSpinButton but uses the same algorithm.
+extern wxSize wxSpinCtrlGetBestSize(const wxControl* spin, int minVal, int maxVal, int base);
+
 } // namespace wxPrivate
 
 // old wxEVT_COMMAND_* constants