Changed all Plugin classes and PluginManager to be sealed, preventing further inheritance. This improves code safety and clarifies the intended usage of these classes.
Replaced hardcoded icon strings with FontSymbols constants in MoreMenu and converter classes for consistency and maintainability. Removed duplicate FontSymbols class from MidiPlayer and updated usages to reference QuickLook.Common.Controls.
Changed plugin priority in MediaInfoViewer to int.MinValue. Updated ViewerWindow.Actions to display more menu items from plugins in order of priority and to avoid duplicate plugins by grouping by plugin name.