Merge branch 'spinctrl-digits'

Improve setting the number of digits in wxSpinCtrlDouble and make it
consistent on all platforms.

See https://github.com/wxWidgets/wxWidgets/pull/2348

Closes #17085.

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; }