Enable nextSearchTerm function for multiple select

The `nextSearchTerm` function can now also be used with Multi-Value select boxes:

![nextsearchtermmultiple](https://f.cloud.github.com/assets/369777/1827974/cca2fb46-7247-11e3-8168-eb85a274067e.png)

If the function is defined, the current search value will be displayed inside the searchbox. It works if `closeOnSelect` is set to true or false. If no function is defined then the select box will behave as usual. By default no function is defined (as for single-value select boxes).

Reference: #1784