Include required headers explicitly in the widgets sample

Don't rely on wx/treebook.h pulling in wx/treectrl.h and wx/textctrl.h,
forward declare or include the headers declaring the classes from these
headers explicitly where needed.

samples/widgets/widgets.cpp

@@ -49,6 +49,7 @@
 #include "wx/numdlg.h"
 #include "wx/textdlg.h"
 #include "wx/imaglist.h"
+#include "wx/treectrl.h"
 #include "wx/wupdlock.h"
 #include "wx/textcompleter.h"