Fix an issue with the custom command : SET_EQUIPPABLE_MODEL

The player-only FORCE_DROP, (ei_id mode, names: FORCE_DROP, DROPSPECIFICEI) and the mixed FORCEDROP, (slot mode, name: FORCEDROP) are now unified into a single MixedCommand that supports both slot: and ei_id: parameters.

FORCE_DROP slot:-1 - to drop item of the entity/player of their main hand slot FORCE_DROP ei_id:MyExecutableItem - to drop the mentionned EI from the player inventory