Comparing two invalid Entities should always return true

Every time a new invalid Entity is constructed, it's assigned a decremental
negative number. Since two invalid Entities always have different negative
numbers, simple Item() == Item() comparision would return false, as internally
only IDs were compared. From logical point of view it makes however sense
for invalid Entities to be considered equal.

As of now, operator==() and operator!=() takes entity validity in consideration,
so Item() == Item() is always true and Item() != Item() is always false.