text.dat string IDs can now be used in/as @-codes, directly

The TEXT:<x> @-code can now be used with the text.dat string ID rather than
the text.dat string number, if preferred.

For convenience and brevity, ALL text.dat string IDs are now valid @-codes
(case-sensitive). While this functionality is redundant with the TEXT: @-code
now, if there are any conflicts between new/existing @-codes and text.dat
string IDs, the TEXT: @-code can be used to overcome such conflicts.

This should help with localization (to other languages) by using common
single-word replacements (Which, Quit, Next, etc.) in text.dat strings.