Remember last used sort when sorting on an unsorted col

Book list: When sorting on a currently unsorted column, use the last
used sort for that column, instead of always sorting in ascending order.
Fixes #1216714 [[Feature request] I'd like Calibre to memorize the sorting order of a column](https://bugs.launchpad.net/calibre/+bug/1216714)
EPUB metadata: Handle multiple title elements when setting titles

EPUB metadata: When changing the title in an EPUB 2.0 file that has
multiple titles, remove the extra titles. Fixes #1211949 [Private bug](https://bugs.launchpad.net/calibre/+bug/1211949)