Sidebar tree navigation

[quarnster]

There's much that is common with #122 here as a tree can be thought of as a nested list in a way.

ST3 (at the time of writing this at least) just supports showing a filtered file system view, but an oft requested feature is to have a sidebar api to allow functionality like a namespace/class browser and to display other hierarchical information.

We should design to allow this wider abstraction from the start.

[karabijavad]

+1

[quarnster]

Removing this from v0.1 milestone, there are far better items to prioritize in getting the editor into a useable state first ;)

[quarnster]

The array prototype code might aid here.

[zoli]

moved to backend repo limetext/backend#107.