Enable visibility toggle of layer groups

[mfisher87]

When a layer group is toggled visible or invisible, all the layers within the group should follow suit. I think a good first iteration is to make it like a "check all" / "uncheck all" button; i.e. there is no group visibility persistent state, the button just sets the state of all the layers within it. The user could then toggle the visibility of one layer within the group if they want to.

Let's say for example we have a layer group that represents 1 year of data, and 12 layers within it represent each month in the year. The user could hide them all with one click on the group visibility button, then re-show the February layer by toggling its visibility back on.