Twig will always use gettext, legacy will use legacy. Twig does not
work with legacy so new ui would break if you would not also set this
option. Instead, just switch the system in use based on the usenewui
config variable.

This reverts commit a4dcf65a.

* Support metadata XML directly in config
Also consider 'xml' as a key, in addition to 'url' and 'file'.
When provided, attempt to load this directly as a metadata XML string.
This is useful when working with dynamic configuration and external polling of metadata updates.

* Test loading XML from a fixed string
No preexisting test in this area
Adjust XML loader to only get global config in file mode - otherwise it is
unused, and it might pollute the test usage.
Test that an actully invalid config throws an exception, but that minimal valid
(albeit useless) XML is accepted.

Explicitly test a file that does not exist, as well as a preloaded one
Virtual test file names intented to be semantic but not collide.