Tentative {virtual_libraries} template support

This should allow the use of {virtual_libraries} in custom column
templates, with good performance, thanks to search caching.

src/calibre/db/cache.py

@@ -1620,6 +1620,18 @@ class Cache(object):
         if cover and os.path.exists(cover):
             self._set_field('cover', {book_id:1})
         self.backend.restore_book(book_id, path, formats)
+
+    @read_api
+    def virtual_libraries_for_books(self, book_ids):
+        libraries = tuple(self._pref('virtual_libraries', {}).iterkeys())
+        ans = {book_id:[] for book_id in book_ids}
+        for lib in libraries:
+            books = self._search(lib)  # We deliberately dont use book_ids as we want to use the search cache
+            for book in book_ids:
+                if book in books:
+                    ans[book].append(lib)
+        return {k:tuple(sorted(v, key=sort_key)) for k, v in book_ids}
+
     # }}}
 
 class SortKey(object):  # {{{

src/calibre/db/lazy.py

@@ -231,6 +231,15 @@ def composite_getter(mi, field, metadata, book_id, cache, formatter, template_ca
             template_cache=template_cache).strip()
         return ret
 
+def virtual_libraries_getter(dbref, book_id, cache):
+    try:
+        return cache[field]
+    except KeyError:
+        db = dbref()
+        vls = db.virtual_libraries_for_books((book_id,))[book_id]
+        ret = cache[field] = ', '.join(vls)
+        return ret
+
 getters = {
     'title':simple_getter('title', _('Unknown')),
     'title_sort':simple_getter('sort', _('Unknown')),
@@ -247,6 +256,7 @@ getters = {
     'series_index':series_index_getter(),
     'application_id':lambda x, book_id, y: book_id,
     'id':lambda x, book_id, y: book_id,
+    'virtual_libraries':virtual_libraries_getter,
 }
 
 for field in ('comments', 'publisher', 'identifiers', 'series', 'rating'):