Emit SidebarDropdownWrapper state

[pasevin]

Summary

It would be useful to be able to listen to the isOpen state of the SidebarDropdownWrapper.

Basic example

const dispatch = createEventDispatcher();
const handleDropdown = () => {
    isOpen = !isOpen;
    dispatch('toggle', { open: isOpen });
};

Motivation

Currently the only prop that we have is propagating downwards via isOpen.
When use interacts with the dropdown we would like to save the state if isOpen into the local store so that we could re-open the relevant sidebar items on the page reload.