Bug fix in "Add signals..."

A very discerning user from Argentina (Thanks!) stubled upon a bug in the "Add signals..." procedure whereby if a sound file name started with a capital letter, the script would not run properly. This is due to the fact that the Praat scripting language does not allow variable names starting with an upper case character.

I fixed the bug in the new release (1.9.6) and now upper case characters are allowed.