Renamer: Avoid listbox get_children

[danirabbit]

There is no more ListBox.get_children () in GTK4

There's one place I didn't replace here and it's the block starting at line 154. I suppose we could iterate over the list and add to a counter to get the number of children? Alternatively if we switched this whole thing to use a ListModel we could get the number of items in the model, but we'd need to add a "new_name" property to Files.File. Thoughts @jeremypw?

[jeremypw]

I thought I had already rewritten this somewhere avoiding getting the number of children but I can't seem to locate it atm. If I can't find it I'll do it again.

[jeremypw]

As we only add rows in one place and never remove them, the simplest solution is to just count them as they are added. Using a model would be more elegant if rows are added and removed adhoc.