added: checking whether an user inserts a correct value of a variable or a function
added: a last column in the variables' and functions' list is resized into the whole space
fixed: some small artefacts on the bitmap in the about dialog
git-svn-id: svn://ttmath.org/publicrep/ttcalc/trunk@20 e52654a7-88a9-db11-a3e9-0013d4bc506e