Rename `zoom_in_no_step` action to `zoom_in_slight`

Same for zooming out. These actions zoom in slightly, compared to the
main zoom actions, so I think this reads better.
Use natural sorting for filenames

Using `g_utf8_collate_key_for_filename()`, filenames can be sorted more
intelligently (e.g. "foo9.jpg" should come before "foo10.jpg").
Closes #1.