[FIX] Tree: Fix selectionChange event parameter and single selection

- Parameter "nodes" of "selectionChange" event did contain inconsistent
data
- In single select mode, setIsSelected still allowed to select multiple
nodes
- Introduced _getNodes to avoid replication of node arrays for internal
read only operations 
- Added unit tests for selection events

BCP: 1472012012
Change-Id: I4e33566035e6bc2035c81b6c35c43c8cdce1d283
Merge "[FIX] Tree: Fix selectionChange event parameter and single selection"