Imported BaseEngine from the DEP.

i18n is left aside for now.
Added initial support for loading template engines.
Added dummy template backend.
Added jinja2 template backend.
Added Django template backend.
Looked up the default template engine in the list of all engines.
Removed private API django.template.loader.find_template.
Removed some uses of global APIs from django.template.loader.
Supported multiple template engines in get_template and select_template.
Added support for explicitly selecting an engine.
Removed private API get_template_from_string.
Removed extraneous arguments in Engine.get_template_from_string.

This aligns the Django Template Engine API with the common template
backend API.
Passed a reference to the current engine when instanciating Template.
Renamed get_template_from_string to from_string.

The shorter name is just as explicit and, well, shorter.
Added a comment about the last use of Engine.get_default().
Supported multiple template engines in render_to_string.

Adjusted its API through a deprecation path according to the DEP.