Added notifications for new objectives to scripted missions

Fixes #6692
Merge pull request #6699 from obrakmann/fix6692_new_objective_notification

Added notifications for new objectives to scripted missions
Add an aspect ratio correction flag to VqaPlayerWidget.
Make the video cancel hotkey configurable.
Prevent mouse events from propagating through a video.
Fix the title of the TD missions panel.
Add support for mission groups.
Add support for briefing videos in the mission menu.
Upgrade TD missions for the new mission chooser.
Upgrade RA missions for the new mission chooser.
Add support for thirdparty missions.
Update ingame briefing panel for new preview dimensions.