Select layers using drop down boxes instead of layer list

[dstansby]

As discussed in #56, it would be better to have a set of drop down boxes for layer selection, instead of relying on the layer list built into napari. This will be a major breaking change, but as long as it's documented I'm fine going ahead since we're still pre-1.0.

[dstansby]

In terms of implementation, I think it would be nice to have a base class that:

• Sets up a Matplotlib figure
• Stores the number of layers that are being visualised on that figure
• Has an abstract method to generate the plot

This class would provide both the figure and the correct number of selectors, and then sub-classes would implement the method that does the actual plotting.

[dstansby]

More thinking, and decided against this, at least for now