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.
Introduces 'Reopen as source code' and 'Reopen as image preview' options in the MoreMenu for SVG and HTML files, enabling users to switch between ImageViewer and TextViewer plugins. Updates translation files to support new menu items in multiple languages and refactors related plugin code for extensibility.
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