Rename wxStaticText::Do[SG]etLabel() to WX[SG]etVisibleLabel()

The names of these methods were confusing because they implied that they
were the actual implementations of the public [SG]etLabel(), while this
wasn't at all the case.

Give them then ames describing what they really do and also update the
comments to hopefully be more clear.

No real changes.

include/wx/univ/stattext.h

@@ -58,8 +58,8 @@ protected:
     // draw the control
     virtual void DoDraw(wxControlRenderer *renderer) wxOVERRIDE;
 
-    virtual void DoSetLabel(const wxString& str) wxOVERRIDE;
-    virtual wxString DoGetLabel() const wxOVERRIDE;
+    virtual void WXSetVisibleLabel(const wxString& str) wxOVERRIDE;
+    virtual wxString WXGetVisibleLabel() const wxOVERRIDE;
 
     wxDECLARE_DYNAMIC_CLASS(wxStaticText);
 };