program: added append operation to program update

new update append operation (in addition to insert, change, and remove)
is similar to insert except adds the line to the end of the program; is
meant to only be used when loading a program from a file (not yet
implemented in the program class); and is triggered by passing a -1 for
the line number to the update function (which the edit box instance will
not do)

the operation enumeration definition (with a new value for the append
operation) was moved from the error list header file to the program
model header file (the error list no longer uses the operation once
the no longer present set change index  function was removed)