- size was stored as index in the combo box
- items were set in .ui, not dynamically built
- QtDesigner doesn't support setting user data to QComboBox items
- compatibility preserved using a trick: any value between 0 and 2 is old way, new way uses -1 and values >= 250
- safe default if value is outside range is read from config file
- fallback on smaller thumbnails if desired size isn't available
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