[INTERNAL][FIX] P13nColumnsPanel : smaller bug fixes

- _getNextItemIndex & _getPreviousItemIndex were simplified
- bug fix in _swopShowSelectedButton as now selected item will be
focused as soon as the "Show Selected" modus is left
- unit test adaption

Change-Id: Iec2630f222da6ff948762d060d5cdf41c0165e09
Merge "[INTERNAL][FIX] P13nColumnsPanel : smaller bug fixes"