Add spans for plots

[azerupi]

This PR adds supports for spans in plots on either axis. This allows for example to highlight a region of interest or draw states in the background of plots.

There are a few points I would like to highlight that would need some feedback:

1. The spans are drawn as plot items and therefore they show up in the legend. The structure in the code makes sense but I'm not sure if the spans should be displayed in the legend. With a lot of spans on a plot this could become quite messy. The name of the spans are displayed in the plot themselves already.
2. The names of the spans are drawn in over them for spans on the x-axis they are drawn at the top-center of the span and for spans on the y-axis they are drawn at the left-middle of the span. When zooming out and the span is too small to display the name it will get truncated and ultimately hidden. I think this looks good and is functional, but this is all very arbitrary so I'm open for other solutions.
3. The spans being plot items means that they also get drawn in the order they are called in the show closure. This means it could be drawn on top of the line plot. This has advantages and disadvantages. I'm not sure if we want to have spans always be drawn in the background of other plot items?

• Closes Add vertical and horizontal spans to plots #148
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