Avoid using mismatched translations for wxWizard buttons

Cache labels for "Next>" or "Finish" button in wxWizard so that their
translations stay consistent throughout wizard's lifetime: previously,
this button could use a label in a different language if the currently
used translations have changed since the wizard creation, as this label
was recreated on every page change, unlike the other labels which were
only translated once in the very beginning.

Closes https://github.com/wxWidgets/wxWidgets/pull/1000

include/wx/generic/wizard.h

@@ -132,6 +132,10 @@ protected:
                 *m_btnNext;     // the "Next>" or "Finish" button
     wxStaticBitmap *m_statbmp;  // the control for the bitmap
 
+    // cached labels so their translations stay consistent
+    wxString    m_nextLabel,
+                m_finishLabel;
+
     // Border around page area sizer requested using SetBorder()
     int m_border;