Extend WMessageResourceBundle::use to allow for directories
WMessageResourceBundle::use may work well for simple translatable strings, but it would be nice for templates if we could put each XHTML fragment in its own file, like:
templates/ ├── footer.xml └── header.xml
Wt::WString::tr("footer") would then yield the contents of
Optionally, we could also treat files with different extensions as plain text instead of XHTML.