Add seashell sprite

Add a seashell sprite in the beginning of the phase.
Move straw polygon to a new folder

Move the file straw-polygon.json to the folder polygon. From now on,
this folder will store the collision shapes for all sprites used in
the game.
Adjust size of the seashell

Add a larger seashell, so that the player is able to jump over the
first platform and get the life drop.