add a new lifecycle event `Restoring` which is set when the editors/viewlets are being restored (start). also in the lifecycle service replace `onDidChangePhase` with `when`

Part of #13305

see https://basarat.gitbooks.io/typescript/content/docs/tips/nominalTyping.html and https://github.com/Microsoft/TypeScript/blob/7b48a182c05ea4dea81bab73ecbbe9e013a79e99/src/compiler/types.ts#L693-L698