Improve chartxyz

This merge request solves some issues I've found when using ChartXYZ. Firstly, I couldn't label the axes, so I've added the ability to do so directly. I know there is a mechanism for pulling labels from plots but not all plot types allow this (for example surface plot does not). I also added the ability to set font sizes and other parameters (on a 4K screen things were rather small!) Secondly, I didn't like the way axes labelling moved around almost randomly, sometimes had one axis's labelling overlapping with another axis's labels, and often labelled edges within the chart. So, I rewrote the mechanism for selecting which edges to label. If you play with a chart, moving it around with mouse or arrow keys (see point 3), you'll see that the labelling is always kept to the outer edges of the 2d-projection of the 3d chart. There are odd limiting cases where the chart is rotated left and right or up and down and, hence I implemented a third feature to allow me better to control rotations, that of allowing the chart to be rotated left/right up/down using the keyboard's arrow keys.