X Series Fixes/Improvements

The X Series is not truly needed by WCartesianChart if every WDataSeries has its own X Series. I reworded the documentation to indicate this.

In addition, the handling of chart updates when the X Series for both the WCartesianChart and WDataSeries is changed was flawed. This pull request includes a fix. I have tested this fix in my environment, but did not test the wide variety of conditions where columns in the model have updated.