tdf#104861: extension list is not a child dlg control anymore

After 989cf68b, 88f31af7 and 3aec456b buttons aren't
embedded in the extension list anymore, so treating it like a
child dlg control just causes Tab key to skip the widget

Change-Id: I4372062c135804d8b51a8640c1e9959f03a617fe
Reviewed-on: https://gerrit.libreoffice.org/49254Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>

include/svtools/extensionlistbox.hxx

@@ -36,7 +36,7 @@ class IExtensionListBox: public Control
 public:
     enum { ENTRY_NOTFOUND = -1 };
 
-    IExtensionListBox( vcl::Window* pParent ): Control( pParent, WB_BORDER | WB_TABSTOP | WB_CHILDDLGCTRL ){}
+    IExtensionListBox( vcl::Window* pParent ): Control( pParent, WB_BORDER | WB_TABSTOP ){}
 
     /** @return  The count of the entries in the list box. */
     virtual sal_Int32 getItemCount() const = 0;