Factor out wxSpinCtrlDouble::DoSetDigits()

Make this part of SetDigits() reusable in order to be able to also use
it in DetermineDigits() now and in SetIncrement() in the upcoming
commit.

No real changes yet.

include/wx/generic/spinctlg.h

@@ -423,12 +423,16 @@ private:
     // Common part of all ctors.
     void Init()
     {
-        m_digits = 0;
+        DoSetDigits(0);
     }
 
     // Update m_digits and m_format to correspond to the given increment.
     void DetermineDigits(double inc);
 
+    // Set the number of digits and the format unconditionally.
+    void DoSetDigits(unsigned digits);
+
+
     wxString m_format;
 
     wxDECLARE_DYNAMIC_CLASS(wxSpinCtrlDouble);