Support workbooks with chart extension

[JanMarvin]

In 2014 someone decided, that having charts[0-9].xml was not enough and introduced chartEx[0-9].xml. These are used with newer chart options in Excel like histograms and box whisker plots.

Unfortunately this breaks the current logic of how we handle charts. Previously we had a nice little data frame in wb$charts where every chart could have a style and a color xml file. When loading we read the chart, color and style into the data frame. This needed a fix, because we can have chart1.xml and chartEx1.xml, but they use color1.xml, color2.xml, and style1.xml and style2.xml. (Oh and ofc they have chartEx1.xml.rels but I assume you already guessed this).

In a PR I'm a about to push in a moment, I have circumvented this by adding new columns to our data frame (chartEx and relsEx). This allows reading and writing the example input.

What needs to be added though is cloning.