Another Undo Issue

Testing has now been completed and I think most of the possible change scenarios have been tested.  There was some more problems with undo operations when at a new line that was not yet reported as inserted.  For some undo operations, a line was not being reported as changed when it should have been.  This occurred when the line of the beginning of the undo change was more than one line away from the current line that was new.

To correct this problem, a check was added if the line of the beginning of the change is more than one line away from the modified (new) line, then the number of lines modified is set to one from zero and since this is the next line, the changed line number is incremented.

Getting into this situation was rather complex, which required an insert of a new line (Control+Enter) in the middle of a line, moving to the end of this new line, deleting the end of this line to combine it with the next line, and then one undo.  This worked so far, but doing undo again is when the problem occurred.  I'm sure there could be more complex sequences that will not be detected correctly, but it's time to move on.  Any new problems that occur will be dealt with as they appear.

There is also a screen update problem that sometime occurs during an undo where a entire line is not redrawn correctly on the screen.  However, this was proven to be a bug in the QPlainTextEdit base class, not in the EditBox class because a simple program using just the QPlainTextEdit also exhibits the same bug (though the QTextEdit class does not).

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