Feature request: visualizing individual z-planes (2.5D)

[melonora]

Currently in napari we do not support the visualization of 3D data. While full 3D support is too much of a task for the near future, we could improve better support for 2.5D. I propose the following:

• Have the possibility of adding a parameter called z-plane to a plotting call. (one parameter z-plane per render_...)
• Internally when dealing with continuous z coordinates have the capability of binning, so lets say I state z-plane=3, this would bin all z coordinates between 2.5 and 3.49 to 3.