Add legends

[milliams]

We should be able to collect the objects being plotted and create a legend. We need to know:

• What objects are plotted
• How to draw an icon for each
• Where to put the legend by default

[Ploppz]

I made a first draft here.
I added legend_svg() to trait ContinuousRepr and implemented it for Line (for now). It uses the Line's LineStyle to determine and draw the icon it should use.
Then, in <ContinuousView as View>::to_svg(), when we loop through the representations we also get their legends, and place them under each other in the top right corner.

Edit: parts of the implementation were apparently in previous commits. For example a way to set the legend in Line: https://github.com/Ploppz/plotlib/blob/wip/src/repr/line.rs#L44

Questions and possible changes:

• Maybe legend_svg should also be a function in the different styles, so that Function::legend_svg and Line::legend_svg can both use LineStyle::legend_svg?
• Make the position of the legends adapt to what is plotted (so that it doesn't collide)?
  • and/or: Make it possible for the user to select the position: one of the corners, and maybe even outside of the plot (I think matplotlib allows this)