Fixed #17085, #24783 -- Refactored template library registration.

* Converted the ``libraries`` and ``builtins`` globals of
  ``django.template.base`` into properties of the Engine class.
* Added a public API for explicit registration of libraries and builtins.

tests/template_tests/syntax_tests/test_if_changed.py

@@ -5,6 +5,7 @@ from ..utils import setup
 
 
 class IfChangedTagTests(SimpleTestCase):
+    libraries = {'custom': 'template_tests.templatetags.custom'}
 
     @setup({'ifchanged01': '{% for n in num %}{% ifchanged %}{{ n }}{% endifchanged %}{% endfor %}'})
     def test_ifchanged01(self):