Bulk trim newdb

Widget + newdb modifications
Bulk trim legacy db
Added the fuzz tweak variable
Replaced the fixed trim

Single and bulk metadata trim modification with tweaked value
Change image export format to jpg
Fixed tweak text for % not available
Cover grid: Add a disk cache for rendered thumbnails
Add a trim cover option ot the bulk metadata edit dialog

Merge branch 'Bulk-trim-covers' of git://github.com/sengian/calibre
Optimize for disk cache

Disable delay rendering and change cache size defaults for optimizing
the use of the disk cache
Nicer controls for changing grid cover size
Show series in grid view tooltip as well
ebook-meta: Ignore the read metadata from file contents setting

ebook-meta program: Ignore the adding books preference that controls the
reading of metadata from file names vs. file contents.
newdb: set_metadata should not allow case changes

set_metadata() was allowing case changes for is_multiple fields, causing
for instance, adding books to change the case of existing
tags/authors/etc. In the old backend it was inconsistent, case changes
were disallowed for most fields, but allowed for a few, for example,
series.
...
Show only the first two version numbers if the third is zero
Better name for sort by cover menu entry
Remove invalid entries from sort menu
Show current aspect ratio when changing cover size
Show traceback when reading metadata from format fails
pep8