Fix multi-selection in data manager (see #11037)

Fix issue with multi-selection. The easiest to validate the PR is

* select 2 images in the data manager (i.e. Tree)
* Go to the script menu and Select the Channel offset script for example
* The 2 ids of the selected images should be displayed.

Screenshot before (check with yesterday's build)
![11037_before](https://f.cloud.github.com/assets/1022396/601909/e74035ec-cc8c-11e2-969f-1f679e142121.png)

Screenshot after (check with Today's build)
![11037_after](https://f.cloud.github.com/assets/1022396/601912/f5232f70-cc8c-11e2-906f-d3f8df909b48.png)


This PR will have to be rebased.