This commit adds a template path provider to separate the process of locating
(cached) paths in extensions from the template engine. The locator is supplied
with a list of paths from the path provider.
Admin templates can now be created in ext/<ext>/adm/style/ and regular
templates go into ext/<ext>/styles/<style>/template/. Extension templates
override regular templates. So if an extension supplies a file with a name
used in phpBB, the extension's file will be used.
A side-effect of this commit: Locator and Provider are now able to deal with
arbitrary levels of template inheritance. So we can expose this through
phpbb_template if we choose to, and allow styles to inherit from inherited
styles.
PHPBB3-10323
