replaced wxWindowGTK::m_isListBox with a virtual function

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@15717 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/gtk1/listbox.cpp

@@ -335,7 +335,6 @@ bool wxListBox::Create( wxWindow *parent, wxWindowID id,
 {
     m_needParent = TRUE;
     m_acceptsFocus = TRUE;
-    m_isListBox = TRUE;
     m_prevSelection = 0;  // or -1 ??
     m_blockEvent = FALSE;