Added plot items:

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

Other changes:

* 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.