Notification Manager Update

[WavesJ99]

Overview
This PR introduces a significant architectural improvement to OpenMCT's notification system by introducing a dedicated NotificationManager class and enhancing the NotificationAPI. The changes provide a more robust and flexible notification system while maintaining backward compatibility.

Key Changes
Introduces a new NotificationManager class to handle core notification logic
Adds support for notification grouping to manage related notifications
Implements a comprehensive priority system considering multiple factors
Adds support for custom notification categories
Introduces persistent notifications that resist auto-dismissal
Improves notification lifecycle management

Benefits
Better separation of concerns between API interface and notification management
Enhanced developer experience with more powerful notification APIs
Improved organization of notifications through grouping capability
More flexible notification categorization and prioritization
Better management of system-critical notifications through persistence

Implementation Details
The implementation splits the existing notification logic into two main components:
NotificationManager: Handles core notification management, grouping, and prioritization
Enhanced NotificationAPI: Provides an improved interface while maintaining backward compatibility

Testing
Added comprehensive test coverage for new functionality including:
NotificationManager unit tests
Enhanced NotificationAPI tests
Group management tests
Category and priority system tests

The changes are fully backward compatible and require no changes to existing notification usage.