- Enable/Disable viewlets by views when context
- Move viewlet enablement/disablement to views service
- Fix - adopt if a viewlet extension is disabled/removed

There are many places in the code that do `import Event, { ...} from '.../event'`.

The FileKind was previously guessed from the collapsability of the item,
extensions now have the capability to set it manually.

Fixes #43216

- Custom view service to get tree item viewer
- Custom view service to register the data provider

Make tree view data providers disposable
On main tree views disposed
- Deregister and dispose data providers