correct errors not being highlighted on start up

the problem occurred because the cursor was not valid, which caused the
extra selections to contain invalid cursor preventing the errors from
being highlighted

a new cursor valid flag member added to the edit box class, which is
initialized to false and only set to true when a cursor move signal is
received, at which time the extra selection list is updated from the
saved error list

a new error list member added to edit box where the received errors list
from the program model is saved (will be needed later anyway to access
the error messages) and the extra selection list will only be updated if
the cursor is valid