Layers should have visible and selectionAPI functions

[manthey]

We can toggle the visibility and interactivity of individual features with the visible() and selectionAPI() functions. It would be nice if these were exposed for layers, so an entire layer (regardless of type) could be hidden or selections disabled (though the interaction option might only be appropriate for feature layers).

Currently, you can hide a layer by changing is opacity, but it still interacts and renders.

[manthey]

A visible() layer function will be added in an upcoming PR.

We currently have an active flag we can pass when we create a layer. The only affect this has is whether the layer has a css class of active, which sets the layer's css to pointer-events: auto (otherwise it is none).

Once a layer is created, we don't have an API for changing the active flag (it would be trivial to have one) other than manually toggling the layer's css class.

Should a selectionAPI state of a layer match that of its active state? For features, we disable event hooks when we turn off the selection API. I imagine that we would cascade that down from the layer, so toggling the layer would do the same. The active state, on the other hand, determines if the layer receives mouse (pointer) events itself, and is orthogonal to our selection API (since that is based on the map receiving mouse events).

Or should selectionAPI be orthogonal to active?

[manthey]

PR #682 adds visible() and selectionAPI() functions to layers. If either is false, then subcomponents (such as features) act as if their values are also false. The active setting on a layer can now be changed (it is independent of selectionAPI).

[manthey]

Added in PR #682.