In his quest to provide all 4-year-olds with high-quality preschool programs, Mayor Martin J. Walsh has floated several ideas for funding, from the radical (selling off City Hall) to the less creative (asking the state for a handout.) Now, in an attempt to finally lock down funds, Walsh announced a seemingly practical proposal Tuesday night: redirecting surplus money from two state Convention Center Fund accounts that are generated specifically by tourism tax revenue raised in Boston.