Property taxes are on the way up in the Souderton Area School District.   The school board Thursday night approved a budget for the next school year that totals about $105,000,000 and includes a property tax increase of almost 5%.   That translates into about $190 for the average homeowner in the district.   School district officials say the next spending plan actually totals almost $100,000 less than the current year budget.   They blame the property tax increase on cuts in state education funding.