Move sidebar to it's own file, refactor to use actions more

We eventually want to add a collapsible sidebar, but there's a few problems to tackle first. One is that the sidebar is a hundred lines, and adding a collapsing mode will make it larger - so we should take it out of the Main component. Secondly is that not everything is using the same component, nor actions so implementing the collapsing would be harder.

Now the sidebar lives in it's own file, and everything is using actions even the "settings" button and others. The code is a bit simplified too and removed some cruft we don't need anymore. The sidebar should be functionally identical.