Fix #1348054 [Private bug](https://bugs.launchpad.net/calibre/+bug/1348054)
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Add documentation and an example editor plugin
Conversion: Fix a bug processing the CSS font shorthand property when the line-height is specified in em or percentage units. Fixes #1348173 [ebook-convert doesn't handle composite font-sizes](https://bugs.launchpad.net/calibre/+bug/1348173)
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