Update supported firmware version for Kobo to 2.8.1

No changes necessary except to bump the firmware version.
Small fixes to get_metadata()
Speed up evaluation of composite columns

Use a ProxyMetadata object that lazily evaluates its fields on demand,
thereby avoiding the overhead of get_metadata() on every composite field
evaluation.
Tentative {virtual_libraries} template support

This should allow the use of {virtual_libraries} in custom column
templates, with good performance, thanks to search caching.
Update taz.de (RSS)
Handle recursive templates in composite columns
Add get_standard_metadata() to ProxyMetadata
...
oops
Use a cache for search query parsing
Fix handling of recursive templates in ProxyMetadata
Update instead of invalidating search caches

Updating is cheap when only a few books are affected.
Invalidate search caches when marked ids change
Ensure that removing books does not cause errors when updating search
caches
fixed antyweb recipe
update Dilbert recipe
Update Scientific American
Merge branch 'master' of https://github.com/intromatyk/calibre

Update Dilbert and Antyweb
Viewer: Refactor the bookmarks manager to make it a little prettier
Complete rewrite of viewer bookmarks manager

Ebook-viewer: Allow re-ordering bookmarks in the bookmarks manager by
drag and drop.