Permission system now provides a default player

The default player has a fixed name and uuid that should never collide with existing players.
It will never have self permission data, and it will always be part of the default groups.

It is useful for anonymous permission test (for instance, listing only the servers that are publicly visible for the ping server list)

pandalib-permissions/src/main/java/fr/pandacube/lib/permissions/DefaultPlayer.java

@@ -0,0 +1,70 @@
+package fr.pandacube.lib.permissions;
+
+import fr.pandacube.lib.util.Log;
+
+import java.util.UUID;
+
+/**
+ * Represents a dummy player in the permission system, that have no specific data, only inheriting from the default
+ * groups.
+ *
+ * The current implementation provides a player named {@code default.0} with an uuid of
+ * {@code fffdef17-ffff-b0ff-ffff-ffffffffffff}.
+ * Trying to set a permission data for this player will log a warning.
+ */
+/* package */ final class DefaultPlayer extends PermPlayer {
+
+
+	// a static UUID that ensure it will not collide with any player, either online or offline or floodgate:
+	// the version bits are set to B (11), that is offline mode (3) + alt (8) account, and alt account counter
+	// set to 0 that is usually impossible due to the counter starting at 1.
+	/* package */ static final UUID ID = new UUID(0xfffdef17_ffff_b0ffL, -1L);
+
+	/* package */ static final String NAME = "default.0";
+
+
+	/* package */ DefaultPlayer() {
+		super(ID);
+	}
+
+	@Override
+	public String getName() {
+		return NAME;
+	}
+
+	public void setGroup(String group) {
+		warnDefaultPlayerSetData();
+	}
+
+	public void addGroup(String group) {
+		warnDefaultPlayerSetData();
+	}
+
+	public void removeGroup(String group) {
+		warnDefaultPlayerSetData();
+	}
+
+	@Override
+	public void setSelfPrefix(String prefix) {
+		warnDefaultPlayerSetData();
+	}
+
+	@Override
+	public void setSelfSuffix(String suffix) {
+		warnDefaultPlayerSetData();
+	}
+
+	@Override
+	public void addSelfPermission(String permission, String server, String world) {
+		warnDefaultPlayerSetData();
+	}
+
+	@Override
+	public void removeSelfPermission(String permission, String server, String world) {
+		warnDefaultPlayerSetData();
+	}
+
+	private void warnDefaultPlayerSetData() {
+		Log.warning(new UnsupportedOperationException("Trying to set permission data of default player"));
+	}
+}