Make wxSpinCtrlDouble::SetIncrement() update digits in wxGTK too

Follow the generic version and increase the number of digits if
necessary.

include/wx/gtk/spinctrl.h

@@ -243,7 +243,7 @@ public:
     void SetValue(const wxString& value) wxOVERRIDE        { wxSpinCtrlGTKBase::SetValue(value); } // visibility problem w/ gcc
     void SetValue(double value)                 { DoSetValue(value); }
     void SetRange(double minVal, double maxVal) { DoSetRange(minVal, maxVal); }
-    void SetIncrement(double inc)               { DoSetIncrement(inc); }
+    void SetIncrement(double inc);
     void SetDigits(unsigned digits);
 
     virtual int GetBase() const wxOVERRIDE { return 10; }

src/gtk/spinctrl.cpp

@@ -643,6 +643,18 @@ void wxSpinCtrlDouble::GtkSetEntryWidth()
     gtk_entry_set_width_chars(GTK_ENTRY(m_widget), wxMax(lenMin, lenMax));
 }
 
+void wxSpinCtrlDouble::SetIncrement(double inc)
+{
+    DoSetIncrement(inc);
+
+    const unsigned digits = wxSpinCtrlImpl::DetermineDigits(inc);
+
+    // Increase the number of digits, if necessary, to show all numbers that
+    // can be obtained by using the new increment without loss of precision.
+    if ( digits > GetDigits() )
+        SetDigits(digits);
+}
+
 unsigned wxSpinCtrlDouble::GetDigits() const
 {
     wxCHECK_MSG( m_widget, 0, "invalid spin button" );