[Savestates] Various fixes and documentation (#277)

- **[Savestates] Fix temporary savestate not saving correctly**
- **[Savestates] Fix savestatehandler being unregistered twice**
- **[Savestates] Improvements to documentation**
- **[Savestates] Fix SubtickGuiScreen being affected by modifier keys
from virtualinput**

Author: ScribbleTAS

src/main/java/com/minecrafttas/tasmod/TASmod.java

@@ -188,7 +188,6 @@ public void onServerStop(MinecraftServer mcserver) {
 		if (savestateHandlerServer != null) {
 			PacketHandlerRegistry.unregister(savestateHandlerServer); // Unregistering the savestatehandler, as a new instance is registered in onServerStart()
 			PacketHandlerRegistry.unregister(savestateHandlerServer.getPlayerHandler());
-			PacketHandlerRegistry.unregister(savestateHandlerServer);
 			EventListenerRegistry.unregister(savestateHandlerServer.getSavestateTemporaryHandler());
 
 			savestateHandlerServer = null;

src/main/java/com/minecrafttas/tasmod/playback/PlaybackControllerServer.java

@@ -42,7 +42,7 @@
  */
 public class PlaybackControllerServer implements ServerPacketHandler {
 
-	private TASstate state;
+	private TASstate state = NONE;
 
 	@Override
 	public PacketID[] getAcceptedPacketIDs() {

src/main/java/com/minecrafttas/tasmod/registries/TASmodPackets.java

@@ -100,7 +100,7 @@ public enum TASmodPackets implements PacketID {
 	 */
 	SAVESTATE_RENAME_SCREEN,
 	/**
-	 * <p>Clears the screen on the client, if it's a savestate screen
+	 * <p>Clears the screen on the client, if it's a savestate screen or a "Loading Terrain" screen
 	 * <p>SIDE: Both<br>
 	 * ARGS: None
 	 */