Fixed sticky pistons destroying unpullable blocks

More specifically, blocks with mobility flag 1 should not be pulled by pistons, they should only be destroyed when a block is pushed into them, and should not be pullable. Previously they'd be destroyed when attempting to pull them.
Also fixes #319

src/main/java/ganymedes01/etfuturum/mixins/backinslime/MixinBlockPistonBase.java

@@ -123,7 +123,7 @@ public boolean onBlockEventReceived(World world, int x, int y, int z, int extend
  Block blockToPull = world.getBlock(x + xoffset2, y + yoffset2, z + zoffset2);
  int metaToPull = world.getBlockMetadata(x + xoffset2, y + yoffset2, z + zoffset2);
 
- if (this.isSticky && !PistonBehaviorRegistry.isNonStickyBlock(blockToPull, metaToPull)) {
+ if (this.isSticky && blockToPull.getMobilityFlag() != 1 && !PistonBehaviorRegistry.isNonStickyBlock(blockToPull, metaToPull)) {
  if (etfuturum$getPushableBlocks(world, x + xoffset2, y + yoffset2, z + zoffset2, oppositeSide, oppositeSide, x + xoffset, y + yoffset, z + zoffset, pushedBlockList, pushedBlockPosList) == 0) {
  world.setBlockToAir(x + xoffset, y + yoffset, z + zoffset);
  } else {