look for templates in srcdir and underlays, first
This entailed changing template_params; it no longer takes the template
filename as its first parameter.
Add template_depends to api and replace calls to template() with
template_depends() in appropriate places, where a dependency should be
added on the template.
Other plugins don't use template(), so will need further work.
Also, includes are disabled for security. Enabling includes only when using
templates from the templatedir would be nice, but would add a lot of
complexity to the implementation.