Improve first character check in CSS identifiers

A CSS identifier can't start with a digit, so when a folder name does,
a '_' character is appended at the beginning of its CSS identifier.

The check for this first character used the isdigit() function, which
takes a char argument, while [self objectAtIndex: 0] returns a
unichar, i.e. a 16-bit unsigned integer. This caused some non-digit
characters to pass this check (e.g. Chinese characters), ending up with
an underscore at the beginning of the folder name.