made catching of lines with undo and redo work properly

reimplemented undo() and redo() functions to control these operations
implemented slot to capture when undo commands are added
now keeps track of the number of modifications on the modified line
the undo and redo key sequences are now handled by the new functions
modified line is not set for document changes caused by undo or redo
  (except for undos that start by moving the cursor)
added new line change signal to the edit box class

edit box now emits a line change signal instead of printing the line
temporarily main window connects to this signal and echos the line
  number and line to the console