Implement composite placeholder tiles

[sandstrand]

Common groups of tiles with functions, such ass entrance doors, stairs and arrow targets should be grouped and placed by a placeholder tile.

The placeholder should output all the necessary tiles with proper naming which works with the component script. In 0.4 we could use the following composite tiles:

• placeholder.stairs.n
• placeholder.stairs.s
• placeholder.stairs.w
• placeholder.stairs.e
• placeholder.target_statue
• placeholder.entrance

Later we could use:

• placeholder.blown_hole
• placeholder.drop_statue
• placeholder.tongue statue.