Add sections and split methods into essential / extended API

For internal classes only in this PR. Some of these are pretty much finished already.

* [ ] Atom
* [ ] BufferedNodeProcess
* [ ] BufferedProcess
* [ ] Clipboard
* [ ] Config
* [ ] ContextMenuManager
* [ ] Cursor
* [ ] Decoration
* [ ] DeserializerManager
* [x] Editor
* [ ] EditorView
* [ ] Git
* [ ] MenuManager
* [ ] PackageManager
* [ ] Pane
* [ ] PaneView
* [ ] Project
* [ ] ScrollView
* [ ] SelectListView
* [ ] Selection
* [ ] Syntax
* [ ] Task
* [ ] ThemeManager
* [ ] View
* [ ] Workspace
* [ ] WorkspaceView
Typo (an extra "the title")