Revamp Tile creation (again)

This breaks down the handling of tile creation even further.
- Introduced a static Tile::override() method to allow overriding the construction class for a specific type of chest. This applies to classes as opposed to save IDs, so you can override Chest::class with MyCustomChest::class and it will take effect for any Chest save ID.
- Removed MCPE stringy save ID constants from public Tile interface. These are now only used for creating saved tiles from a stored chunk, and saving them.
- Renamed Tile::registerTile() to register()
- Tile::create() and Tile::createFromItem() now accept a class parameter instead of a stringy save ID.
- Tile::create() and Tile::createFromItem() were changed to throw \InvalidArgumentException on unknown/unregistered tile types. They also now never return null, but always (except in exception cases) return an object which is an instanceof the base class specified.

src/pocketmine/block/ItemFrame.php

@@ -85,9 +85,7 @@ class ItemFrame extends Flowable{
 		$this->facing = $face;
 
 		if(parent::place($item, $blockReplace, $blockClicked, $face, $clickVector, $player)){
-			if(($tile = Tile::createFromItem(Tile::ITEM_FRAME, $this->getLevel(), $this->asVector3(), $item)) !== null){
-				$this->level->addTile($tile);
-			}
+			$this->level->addTile(Tile::createFromItem(TileItemFrame::class, $this->getLevel(), $this->asVector3(), $item));
 			return true;
 		}