Add more plot items

[EmbersArc]

I'd like to add the following items:

• Arrows, also called "Quiver plots" in matplotlib etc.
• (Convex*) polygons
• Text
• Images

Additionally it would be nice to make the following changes to existing plot items:

• Make HLine/VLine into PlotItems as well.
• Add a "fill" property to Line so that we can fill/shade the area between a line and a horizontal reference line.
• Add stems to Points, which are lines between the points and a horizontal reference line.
• Allow using .. when specifying ranges for values generated by explicit callback functions, as an alias for f64::NEG_INFINITY..f64::INFINITY
• Allow using ranges with exclusive end bounds for values generated by parametric callback functions to generate values where the first and last value are not the same.

*I also have an implementation of the ear-clipping algorithm lying around that can be used to tessellate non-convex polygons. It's relatively few lines of code, but it's not the most efficient algorithm with O(n²) time complexity. I could add this to the plotting library, but maybe it's something for egui in general?

I've already implemented those things and will open a PR once #410 is merged

new_items.mp4