[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
CR-Id: 012006153200015354562014
Merge "[FIX] Tree: Fix selectionChange event parameter and single selection" into rel-1.26
[FIX] ODataModel: getLength should return 0 while refreshing

We recently made change to make getLength return an estimated length, if
the final length is not known yet. However this should only be done,
once the server returned data for the first time, not while waiting for
the initial response.

BCP: 0020079747 0001145175 2014
Change-Id: Icb09c45f51e9c71b109b255df38d5293363e29c9
CR-Id: 012006153200015516182014
Merge "[FIX] ODataModel: getLength should return 0 while refreshing" into rel-1.26