Replace not-fully-initializing DTO constructors

There were some constructors in the DTOs which allowed construction of
DTO which weren't fully initialized. These constructors have been made
private, as they are still used in the 'fromJson' methods. Additionally,
a constructor with all required parameters to fully initialize the
class has been added.

Additionally, the Loader class has been modified, since it no longer can
assume it is able to default construct the parameter type. The parameter
is now stored as an optional.

Closes #15

core/include/JellyfinQt/dto/baseitemdto.h

@@ -69,7 +69,20 @@ namespace DTO {
 
 class BaseItemDto {
 public:
-	BaseItemDto();
+	BaseItemDto(							
+		QString jellyfinId,																																							
+		Video3DFormat video3DFormat,																																					
+		PlayAccess playAccess,																																													
+		QSharedPointer<UserItemDataDto> userData,																																																			
+		VideoType videoType,																																			
+		LocationType locationType,			
+		IsoType isoType,																																											
+		ImageOrientation imageOrientation,																													
+		ChannelType channelType,			
+		ProgramAudio audio,																			
+		QSharedPointer<BaseItemDto> currentProgram		
+	);
+
 	BaseItemDto(const BaseItemDto &other);
 	
 	/**
@@ -1700,8 +1713,13 @@ protected:
 	std::optional<bool> m_isPremiere = std::nullopt;
 	QString m_timerId;
 	QSharedPointer<BaseItemDto> m_currentProgram = QSharedPointer<BaseItemDto>();
+
+private:
+	// Private constructor which generates an invalid object, for use withing BaseItemDto::fromJson();
+	BaseItemDto();
 };
 
+
 } // NS DTO
 
 namespace Support {