Mayor Rahm Emanuel Wednesday unveiled what is sure to be a major plank of the platform he hopes will win him a third term — free preschool for all 4-year-olds in Chicago. At the same time, a political action committee linked to the Chicago Teachers Union dropped the first independent expenditure in the mayoral campaign to oppose Emanuel’s re-election. Activists from the Southwest Side planned to gather at City Hall Thursday to call for more mental health services in Hispanic communities.