Undeprecate `showNotification()` and add `NotificationType`

Created by: lguychard

Fixes #1471 (closed)

This PR removes the (unintended) deprecation of window.showNotification() (see this comment) and extends it to accept a NotificationType.

Notable changes:

• The internal MessageType enum has been removed in favour of NotificationType.
• Removed unused NotificationsService.logMessages & LogMessageParams interface (it was never exposed to extensions): again, while we should introduce a logging interface for extensions, it should be distinct from the notifications service.
• Here's how notifications look in the browser extension and the web app: