Error Action Enabling/Disabling

Now with actions to move the cursor to the next or previous error, these actions should only be enabled when there are actually errors.  To accomplish this, an errors available signal was added to the edit box class.  The update errors slot was modified to determine when the errors available condition changes by comparing the before and after error list emptiness status.  Several problems were found and corrected while testing the new signal.

In the update errors slot, when there were no errors in the incoming errors list, a Qt error occurred because bad indexes were used in accessing the errors list.  This was due to using the start and end change indexes when they were not valid.  A check was added for when the incoming error list has no errors to prevent this.

The second problem occurred when there were no errors and the next or previous error key shortcuts were used.  This was due because the Ctrl+. (period) and Ctrl+, (comma) were not intercepted since the errors actions were disabled, causing these keys to be treated as a regular period and comma, which was inserted into the program.  Checks were added to the key press event handler to catch and ignore the period and comma keys when the Ctrl modifier was active.

Finally, during testing, an unrelated problem was found with the detection of changed lines when comparing RPN lists that contained errors.  Previously this comparison (equality operator function) compared the error column and length and reported equality if they were the same.  This caused a problem were sometimes the error was not displayed correctly in the modified line (or not at all).  The equality function was modified to report no match if either RPN list has an error.

[commit 352667221f] [commit 15084235a2]