Plot 3D symbols via plot3d in animations (via events)

See this [forum pos](https://forum.generic-mapping-tools.org/t/perpective-view-gmt-events/3417)t for background. In the design of **events** we only considered plotting symbols via **plot**, and then later we added the capability of using **meca**, **coupe**, and **velo**. However, 3D symbols via **plot3d** were left out.

I think there are various ways we can extend events to handle symbols offered by **plot3d**. Since **plot** and **plot3d** both can accept z-ranges in **-R** and **-p** and **-JZ** as well, it is not enough to check for those options. Furthermore, **plot3d** expects a z-coordinates as part of its input. Hence, I think we need to tell **events** that we are selecting from the 3-D symbols in **plot3d**. Some possibilities:

1. The only unused upper-case letter in event is **-I**. So it could be used to flag 3-D mode.
2. We could add a modifier to a suitable option to flag this effect.
3. We could let **-Z** (used for **meca** etc symbols) flag 3D by giving it no arguments.

I think the last suggestion makes most sense. **Z** is a better mnemonic than **I** and it should be simple to just let **-Z** with no arguments mean two things:

1. Call **plot3d** instead of **plot** when doing the plotting (the **-S** argument etc are passed as string options anyway)
2. Expect an extra input column for the z-coordinate

Let me know what you think of this plan for enhancement.