display error messages in the status bar

added labels to the main window status bar to display the current line
  number and column, along with an error message if the line has one
added new create status bar function to main window
added new update status bar slot to main window
added new cursor changed signal to edit box
added access functions to edit box for line number, column and message
added new find index function to error list that returns -1 if no error

note that widgets must be added to the status bar before the show
message function is used, otherwise the widgets will not appear; so a
status ready flag was added to prevent the initial "program loaded"
message from being displayed