Require ability to group buttons into a button group

[manzanotti]

Currently, all layers add a single button to the toolbar. This is fine, as they are all separate concepts.

However, going forward, I want to add bus gates, which are a variation of modal filters, so it would be good to only have one of them in the toolbar at any one time.

This requires the ability to specify which group a layer is in, as well having a way to bring up the other options for that button (I'm thinking of on hover, right-click and long-press might be good options), and allowing the user to choose a different layer from that group and swapping that button to be the active one in the toolbar.