SolaraViz: Allow rotating markers

[EwoutH]

Allow rotating markers, so you can (for example) see the heading of an Agent.

For example, in BoidFlockers, the current visualisation is:

However, if you can rotate the markers, you can actually see where the birds are going.

[quaquel]

I think this is tied to various open issues such as #1278, #1354, #845, and #2149.

The first step in my view is to refactor ContinuousSpace and they reuse the ideas from CellAgent (#2292) for one or more mixins with movement methods and attributes valid for contiuous spaces. This is in my view a target for 3.1 as part of the stabilization of cell spaces.