Fixing theme bug where dir must exist in core + expanded "default" fallback

[weaverryan]

Hiya!

I noticed two issues with the theming system:

1. If you use $configuration->setTheme('my_theme');, Twig would ALWAYS try to look
   for a lib/Templates/my_theme directory (inside rst-parser itself), which would cause an exception. Setting themes was unusable.

2. There are two reasons to put "override" templates inside a library that uses rst-parser:
   A) to override individual pieces (e.g. paragraph.html.twig)
   B) to override layout.html.twig for themeing

Previously, if you had 2 themes (theme_foo and theme_bar), each with their own layout.html.twig file, there was no place to put the overrides for part (A): the system would ONLY look inside the active theme directory for ALL overrides (so paragraph.html.twig would need to live in both directories, so that it was found when you switched).

There was already a "fallback" concept: look inside the user's "theme" directory, then fallback to the default directory in core. I expanded that to always look for the theme directory first, but then look for a directory called default in all template directories. This allows you to put all your "component" overrides in a default directory, then ONLY your layout.html.twig in the theme directory (though you could still override "components" there if you need to).

Test included and is BC... because themes weren't working properly yet :).

Cheers!