* Changed LastPlayed field from string to nullable DateTime
Added ApplicationData.LastPlayedString property
Added NullableDateTimeConverter for the DateTime->string conversion in Avalonia
* Added migration from string-based last_played to DateTime-based last_played_utc
* Updated comment style
* Added MarkupExtension to NullableDateTimeConverter and changed its usage
Cleaned up leftover usings
* Missed one comment
