Civic Effect: added AllowsUnitClassImmigration and made immigrant number able to go down

Adds the ability to turn on and off if a UnitClass should appear on the dock
    Default is always allowed
    No unit can appear on the dock unless it is also allowed by AllowsUnitClasses

iIncreasedImmigrants is now able to go both up and down
    This allows civics to gain a bonus, which can be removed again
    When removing units on the dock, it starts from the right

resolves #8