Support Metric Names with "special characters" for Column Chooser Drag&Drop

!!! please check if this right way to do it.
... column names containing special characters (here greek "average"
sign) did not work in Drag&Drop (even though they should be part of
Latin-1 (after changing in Mime to UTF-8) not only the greek characters,
but also "TM" works without special treatment)
!!! is this valid for all platforms ?!!!

(cherry picked from commit b4d1a7d3041beec686c7262cfa65bac8cf6e2179)
Merge pull request #906 from Joern-R/pull1

Support Metric Names with "special characters" for Column Chooser Drag&D...