Revert "Issue #1954892 by dawehner, tim.plunkett, David_Rothstein, effulgentsia: Replace 'theme callback' and hook_custom_theme() with a clean theme negotiation system."

This reverts commit fc04601c.

Issue #1954892 by dawehner, tim.plunkett, David_Rothstein, effulgentsia: Replace 'theme callback' and hook_custom_theme() with a clean theme negotiation system.

Issue #2074037 by Wim Leers, effulgentsia: Add drupalPostUrl() — drupalPost()/drupalPostAjax() are for forms only, D8 JS performs non-form HTTP requests.

Issue #2014011 by kgoel | DmitryDrozdik: Replace drupal_container() with Drupal::service() in the shortcut module.

Issue #1829224 by alexpott, pfrenssen, vijaycs85, lirantal: Convert the 'theme_default()' variable to CMI.

Issue #722650 by alexpott, tim.plunkett, arpeggio: Fixed Shortcut doesn't properly clean up or store menu links.

Issue #916388 by disasm, podarok, dawehner, andypost, amateescu, Dave Reid: Convert menu links into entities.

Issue #1269742 by katbailey, beejeebus, pwolanin, g.oechsler, ksenzee: Make path lookup code into an pluggable class.