Use a configuration event that can answer if a configuration has changed or not given a resource and override identifier.
Requested configuration can be full key or section

- CLean up: Separate models and interfaces

- API to get a value given a key
- API to update value smartly
- Fine grained configuration change event
- Smart reload API
- Remove unnecessary APIs

This reverts commit 838d4a33.

Use WorkspaceFolder instead of URIs in IWorkspaceContextService

- Provide folder specific keys