Add retain method to ListStore

[ranfdev]

Motivation:
People often endup doing

for item in list_store.iter()
  if item.unwrap().name() == 'Giorgio' {
    list_store.remove(&item)
  }
}

But that unwrap is going to panic, because we added checks to prevent that an iterator over a list is mutated during iteration.

retain is just a nice alternative to provide an easier to use way to remove items while iterating the list.

list_store.retain(|item| item.name() != 'Giorgio')

It will also remove consecutive items with a single splice call, so If the user removes every item, retain doesn't spam items_changed signals