Add onColumnsSet event triggered when setColumns is called

Currently SlickGrid triggers no event when columns are hidden/shown, for example by the columnpicker. Such an event is useful if we want to persist the UI's state.

I suggest this two-line change that adds a new onColumnsSet event, triggered every time setColumns is called, with an empty args paramater, in the vein of the other columns-related events (user can call getColumns).