Support for a tabbed-editor #88

It would be useful to have multiple editor tabs open at once. Add a minimal tab-bar at the top of the editor page that allows opening additional pages as tabs.

Clicking on the tabs duplicates the entire editor instance (so they have separate editor services).

Will need to figure out what to do about the route in the address bar. Makes sense to have it open to the "main" (first) tab in the tab bar, but will need to account for what happens when that parent closes.