Move wxEVT_TEXT generation to wxSpinCtrlGTKBase::GTKTextChanged()

This is more consistent with GTKValueChanged() and allows making the
change in the upcoming commit.

No real changes yet, this is just a refactoring.

include/wx/gtk/spinctrl.h

@@ -72,6 +72,7 @@ public:
     virtual bool GTKOutput(wxString* text) const = 0;
 
     virtual void GTKValueChanged() = 0;
+    void GTKTextChanged();
 
 protected:
     wxSpinCtrlGTKBase();