Add ViewExecutable::getStylePlugin and ViewExecutable::getRowPlugin as public methods

Problem/Motivation

\Drupal\views\ViewExecutable::$style_plugin and \Drupal\views\ViewExecutable::$rowPlugin
should have corresponding methods, one could call.

Once we do that, we can also mark the public properties as deprecated

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes