[SHOWSTOPPER] Fix for (EXT-4704) Add maturity icons to Prefs -> General and...

[SHOWSTOPPER] Fix for (EXT-4704) Add maturity icons to Prefs -> General and (EXT-4705) Show maturity icon on Region/Estate tab.
- Made changes to combobox onItemSelected() to fix (EXT-6269) typing a location into the nav bar always sends you to the first location you typed in.
- Replaced LLIconsComboBox pointers with base class pointers LLComboBox.

Reviewed by Richard Nelson https://codereview.productengine.com/secondlife/r/29/

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branch : product-engine