Bugfix/#64: Empty regions verification optimization

[GyD]

This pull request relates to BOSA_0100-358 (JIRA issue) and contains Yves's (@GyD) attempt to improve the performance of Kiso.

• The has_{region} functionality is kept so theme using it still can use it. Functions used to check if region is empty are kept but adapted so theme using it still can use it.
• A per region variable page_{region} is introduced for future themes, that variable store the rendered output of the page.region so it can be called in theme without re-redering the region again,
• A function _kiso_render_region() is introduced to avoid calling \Drupal::service('renderer')->render($region); each time but also not mark the region as rendered since the region is not passed by reference. This way if you need to alter region content after kiso_preprocess_html() and kiso_preprocess_page() it still behave as Drupal core.

[rutiolma]

Looks good to me

[smillart]

Just a comment added regarding the kiso_preprocess_html() function that contains a potentially unused $variables['page_tools'] variable. @GyD Could you please take a look? Thanks!