[css3-ui] Support cropping in the middle of texts using 'text-overflow'

As I already proposed earlier[1] there should be a way to allow truncating
text contents in the middle.
I was asked to add the current proposal to the W3C wiki to allow further
discussion on this. So here it is:

http://www.w3.org/wiki/Text-overflow_middle_cropping

There are definitely still some unconsidered parts, so it would be great to
hear some opinions on this.

Sebastian

[1] http://lists.w3.org/Archives/Public/www-style/2012Apr/0336.html