Fix UI problem navigating FileSelector using autocomplete and keyboard

With this fix, when the list of possible completions for a filename
is regenerated, we deselect the previously selected item. So the new
list appears without any item selected by default.

src/ui/combobox.cpp

@@ -191,6 +191,7 @@ void ComboBox::removeAllItems()
     delete *it;
 
   m_items.clear();
+  m_selected = -1;
 }
 
 int ComboBox::getItemCount() const