Add support in FF for double line-through

Follows CSS3 standards. Unfortunately, only FF implements this so far.
Merge pull request #758 from peitschie/style-double-strike

Add support in FF for double line-through
Add font-size layout tests
Implement support for style:text-position

style:text-position defines sub and super-script text properties.
Ignore ODT files in the tests directory

ODT files are generally created in this directory as a result of running
the layout tests from the source directory.