changed how custom elements change to player if they are walkable

changed how custom elements change to player if they are walkable

Before this change, if a walkable CE changes to the player, the player
is placed on the CE, which will be kept under the player (so it does
not really change to the player, but just adds the player). This is
probably not the expected behaviour when changing a CE to the player
(but may make sense when using the "extended change target", placing
the player on one of the surrounding elements if they are walkable).

This change fixes this behaviour, so by default a CE that "changes to
the player" is replaced by the player, and not just adds the player.
However, a config option was added to the player element to explicitly
request the previous behaviour (which is used by the "Zelda II" level
set, for example).