Ask the user which column contains text

It's more obvious than silently taking the first one.
Update translations.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/r-temis@536 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Print DTM if it exists but is empty

This makes it clearer to the user that there are no documents
or no terms (the latter can happen if on selects the wrong column).

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/r-temis@537 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Make try() silent when testing for invalid characters

There's no point in printed the error since we show a more
explicit dialog.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/r-temis@538 edb9625f-4e0d-4859-8d74-9fd3b1da38cb