[meta] Refactor template_preprocess()

This is a meta issue to track proposed changes to template_preprocess().

Main motivations:

• Some of what template_preprocess() does no longer makes sense now that we are converting theme_*() functions to templates - #1938430: Don't add a default theme hook class in template_preprocess()
• Performance should be improved.

Issues:
#1938430: Don't add a default theme hook class in template_preprocess()
#1982024: Lazy-load Attribute objects later in the rendering process only if needed
#1982020: Add static caching to drupal_html_class()