Add sections and split methods into essential / extended API

For internal classes only in this PR. Some of these are pretty much finished already.

* [x] Atom
* [x] BufferedNodeProcess
* [x] BufferedProcess
* [x] Clipboard
* [x] Config
* [x] ContextMenuManager
* [x] Cursor
* [x] Decoration
* [x] DeserializerManager
* [x] Editor
* [x] EditorView
* [x] Git
* [x] MenuManager
* [x] PackageManager
* [x] Pane
* [x] PaneView
* [x] Project
* [x] ScrollView
* [x] SelectListView
* [x] Selection
* [x] Syntax
* [x] Task
* [x] ThemeManager
* [x] Workspace
* [x] WorkspaceView
I thought packages have to still use it to register deserializers for pane items, such as image view, settings view, archive view.