Closes#37
Almost completely refactored the UI parts outline.nvim to use a Sidebar
object that implements an outline window. In init.lua, we can then store
a table of the outline for each tabpage ID.
When tabs are closed the outline is closed and sidebar reset
responsibly.
This simplifies `init.lua` quite a lot, making it the highest level
control center for the outline elements.
All lua APIs and commands should work as before.
