Finish custom shield script

There are still things to do to finish the code of the first shield:

[x] Separate Eldran sprite from shield sprite (images and dat files).
[x] Code collision events for the shield (this will be used for enemies, including projectiles).
[x] Test if hero is protected against enemies when using the shield. There may be issues.
[x] Decide if we will use the direction fix (like in Minish Cap) or not (like in Link's Awakening).

solarus-games / children-of-solarus