Implement logic to identify different widgets with different layer numbers/types

[dstansby]

Each widget that makes a specific plot takes specific number of layers (e.g. 1, 2, or this could be any number > 1) of specific types (e.g. image, points, but this again could allow for multiple types) as input. We need a nice way of storing this information in the widget class.

For number of layers there's the n_layers_input class attribute:

napari-matplotlib/src/napari_matplotlib/scatter.py

Line 17 in 4b46774

n_layers_input = 2

but this doesn't allow for the situation where a range (e.g. > 1) layers are acceptable as input.

For layer types, I think a new class attribute called layer_input_types that lists acceptable input types would be good to add.

Eventually we can add this to API documentaiton, so it's super easy for users to see which types and what numbers of layers are used by each widget.