Fix support of 20-digit numeric seeds

Minetest 64 bit seeds were being listed signed instead of unsigned in some places.
E.g. with seed 14105561135541706857, "Copy Seed" used to put -4341182938167844759 in the clipboard

src/main/java/amidst/gui/main/Actions.java

@@ -248,7 +248,7 @@ public class Actions {
 	public void copySeedToClipboard() {
 		ViewerFacade viewerFacade = viewerFacadeSupplier.get();
 		if (viewerFacade != null) {
-			String seed = "" + viewerFacade.getWorldSeed().getLong();
+			String seed = viewerFacade.getWorldSeed().getLongAsString();
 			StringSelection selection = new StringSelection(seed);
 			Toolkit.getDefaultToolkit().getSystemClipboard().setContents(selection, selection);
 		}
@@ -260,7 +260,7 @@ public class Actions {
 		if (viewerFacade != null) {
 			BufferedImage image = viewerFacade.createScreenshot();
 			String suggestedFilename = "screenshot_" + viewerFacade.getWorldType().getFilenameText() + "_"
-					+ viewerFacade.getWorldSeed().getLong() + ".png";
+					+ viewerFacade.getWorldSeed().getLongAsString() + ".png";
 			File file = dialogs.askForScreenshotSaveFile(suggestedFilename);
 			if (file != null) {
 				file = appendPNGFileExtensionIfNecessary(file);

src/main/java/amidst/mojangapi/world/SeedHistoryLogger.java

@@ -72,9 +72,9 @@ public class SeedHistoryLogger {
 	private String getSeedString(WorldSeed worldSeed) {
 		String text = worldSeed.getText();
 		if (text != null) {
-			return worldSeed.getLong() + ", " + text;
+			return worldSeed.getLongAsString() + ", " + text;
 		} else {
-			return worldSeed.getLong() + "";
+			return worldSeed.getLongAsString();
 		}
 	}
 

src/main/java/amidst/mojangapi/world/WorldSeed.java

@@ -72,17 +72,23 @@ public class WorldSeed {
 	private final String text;
 	private final WorldSeedType type;
 	private final String label;
+	private final GameEngineType gameEngineType;
 
 	private WorldSeed(long seed, String text, WorldSeedType type, GameEngineType game_engine_type) {
 		this.seed = seed;
 		this.text = text;
 		this.type = type;
+		this.gameEngineType = game_engine_type;
 		this.label = type.getLabel(seed, text, game_engine_type);
 	}
 
 	public long getLong() {
 		return seed;
 	}
+
+	public String getLongAsString() {
+		return (gameEngineType == GameEngineType.MINETEST) ? Long.toUnsignedString(seed) : Long.toString(seed);
+	}
 	
 	public String getText() {
 		return text;

src/test/java/amidst/mojangapi/world/testworld/TestWorldDeclaration.java

@@ -93,8 +93,8 @@ public enum TestWorldDeclaration {
 				"testworld",
 				"storage",
 				recognisedVersion.getName(),
-				"" + worldSeed.getLong()).toFile();
-		this.directoryString = RESOURCE_PREFIX + recognisedVersion.getName() + "/" + worldSeed.getLong() + "/";
+				worldSeed.getLongAsString()).toFile();
+		this.directoryString = RESOURCE_PREFIX + recognisedVersion.getName() + "/" + worldSeed.getLongAsString() + "/";
 	}
 
 	public RecognisedVersion getRecognisedVersion() {