ENH: Add mesh viewer

Adds a `vispy.plot.mesh` plotter. I tried to simplify `MeshData` a little bit in the process.

Ready for review/merge.
It's not very `vispy`-like for `meshdata.vertices()` to be a method instead of a property. The problem with making it (and related vars) a property is there are other things like `faces` that have an argument `indexed`. I'd rather make all these of the form `get_vertices(...)` to mirror their siblings `set_vertices(...)`, to make it clearer that they aren't properties. It would have tripped me up less in working with the `MeshData` class.