Add convenient wxFileName::GetAbsolutePath() wrapper and use it

This wrapper simply combines the calls to MakeAbsolute() and
GetFullPath(), but using it results in shorter and more clear code, so
it seems to be worth having.

samples/stc/stctest.cpp

@@ -601,8 +601,7 @@ void AppFrame::CreateMenu ()
 
 void AppFrame::FileOpen (wxString fname)
 {
-    wxFileName w(fname); w.MakeAbsolute(); fname = w.GetFullPath();
-    m_edit->LoadFile (fname);
+    m_edit->LoadFile (wxFileName(fname).GetAbsolutePath());
     m_edit->SelectNone();
 }