Bug 8622: Fix theme fallback

Bug 8622: Fix theme fallback

This patch does the following:
1) Enables fallback for includes between different themes and different
   languages (with the exact same precedence as for .tt files)
2) Enable fallback for XSLT files between different themes and different
   languages (again, same precedence)
3) Change the semantics of the TT [% themelang %] variable so that it always
   refers to the preferred theme and language, rather than the fallback
   theme/language. As a result, all themes must include all javascript,
   css and image resources they use.

Note that these changes actually have no impact whatsoever on an
installation where the default (prog) themes are in use.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Mason James <mtj@kohaaloha.com>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>