Fix regression where multiple coloring rules were used for the same column.
Merge branch 'master' of https://github.com/cbhaley/calibre

Fixes #1340578 [1.44 breaks column coloring](https://bugs.launchpad.net/calibre/+bug/1340578)
Edit Book: Fix regression that caused crash while syntax highlighting CSS stylesheets that contain url() tokens without enclosing quotes