[REF] util.views: refactor converter code, regroup views helpers

[andreabak]

Overview of changes (see commits for details):

• refactored BootstrapConverter code, separating conversion code and operations from bootstrap-specific code. This way other kind of converters can be implemented, reusing pre-defined ElementOperations into lists (eg. ReplaceClasses/RenameAttributes/etc.)
  • this will also be useful in the (near) future to consolidate into util similar helpers in PS custom-util, commonly used to quickly fix studio and COW views (without all the manual lxml operations boilerplate)
• generalized helpers and moved from odoo/upgrade base/16.0/pre-90-convert-bootstrap5.py so that they can be reused for other similar kind of operations using converters (ie. apply converter to all html fields / templates, etc.) (upgrade PR: odoo/upgrade#5431 )
• re-organized views-related helpers into a subpackage util.views (converter, bootstrap, records)
• misc fixes and tweaks