No real changes yet, just refactor the code to always connect these
signals handlers and not just when using non-decimal base.
Add wxSpinCtrl::GTKInput() and GTKOutput() virtual methods to handle
these signals appropriately depending on the type of the control.
This will allow further customizing conversion to/from string in the
upcoming commit and will also make it possible to handle UI locale
different from the C locale in the future.
3ac3763705