Fix misparsing of \textgreek without polyglossia

This is the first part of bug #8553. \textgreek was interpreted as the
polyglossia version even if polyglossia was not used. Now it is not recognized
at all and converted to ERT, but this ensures at least correct LaTeX output.
Merge remote-tracking branch 'github/build-bot-master' into bleeding-edge-master
2014.12.30-17.00