Potential duplicated event handlers for Combobox

[brunano21]

If developers re-initialize the Combobox's list and call Combobox._initList() function, previous List's event handlers are not destroyed compromising the logic of the Combobox widget itself. In fact, it may happen a certain callback could run multiple times.

Combobox should care of them, destroying/removing them before any following Combobox#_initList() call.