KHolidays: New HolidayRegionSelector widget for clients to allow users to configure what Holiday Regions to use.

The widget provides a number of modes and features to fit most use case scenarios I can think of:
* A display only mode
* A single Holiday Region selection mode
* A multiple Holiday Region selection mode with optional secondary selection for Days Off / preferred Holiday Region
* A search line
* Ability to modify level of detail displayed when space is limited
* Qt Designer plugin

Future features planned include a pop-up with details of all holidays defined in a region, and better region naming once holiday types are implemented.

See ReviewBoard http://svn.reviewboard.kde.org/r/5518/