program: now stores text of lines with errors

a new text variable was added to the line info structure, which is used
to store the text of a line with an error, and is used when line of text
is requested since there is no code stored for the line; this variable
is cleared when the line does not have an error

when a line is changed or inserted and the line has an error, a program
change signal is no longer sent (only the edit box changes and inserted
lines and it already has the text for lines with an errors)

however, when a line is appended, a program change signal is always sent
regardless of whether it has an error (only the program will append
lines and the edit box needs the text for all lines including those with
errors)

the program changed slot of edit box was modified to no longer check for
lines with errors (by checking if the line text string was null) because
the program model attached no longer sends lines with errors (if the
change originated from the edit box)