[REF] util.views_convert: simplify ElementOperation and EtreeConverter

Simplified ElementOperation class, removing unnecessary features, such
as `.op()` method.
Made `xpath` attribute part of the base class, subclasses must honor
it to restrict their scope of operation.

Reworked EtreeConverter class methods, generalized to work with any
kind of converter operations (not just Bootstrap conversions).

Also made docstrings private (excluded from online docs).

Author: andreabak