Merge branch 'master' of https://github.com/cbhaley/calibre

Searching: Fix escaped parentheses not being correctly handled when
parsing search queries.

Match books on device: Fix parentheses in search expression not being
escaped. Fixes #1255671 [Match Books dialogue doesn't like parentheses](https://bugs.launchpad.net/calibre/+bug/1255671).

Match books on device: Fix thumbnail in device metadata cache not being
updated when matching books. Fixes #1255676 [[ENHANCEMENT] Match Books to add covers](https://bugs.launchpad.net/calibre/+bug/1255676).
Fix for bug #1256198