select() may generate an identifier that is unusable

When using the `select()` function, the provided path cannot necessarily be used for a subsequent `select()` (or `getattr()`, etc) and will not properly select any items.

This appears to arise because the XPATH index syntax is slightly different than the `path_to` syntax use different semantics.

Specifically, the current `to_path` syntax uses the index of the element in the child widgets list.
https://github.com/tito/telenium/blob/52e30bad3025c31749e9bab50a4bc5b62467679e/telenium/mods/telenium_client.py#L107-L114

However, the XPATH syntax uses the index into the list **of previously filtered items**. This means that if any items are filtered out, the index used by XPATH and the index into the Kivy child list are no longer equivalent.

Start in telenium/mods/telenium_client.py at lines 107-114, where the path and child-widget index are assembled. Compare the child-widget indexing used by to_path with the filtered-item indexing required by XPath, then trace select() and related getattr() handling. Done means a path returned by select() remains usable for subsequent selection or attribute lookup.

Written by the indexing model from the issue text.