Selection list implementation and change element title/description functionality.

In the first commit a list of ModelEntities is added to the model in order to keep track of the selected elements. Implemented as we talked with  @anserran prior to the meeting, today.
Keep in mind that this is necessary in order to be able to add functionality to the Tween Edition Widget.

From the second to the last commit, the "change element title/description" functionality is added to eAdventure Mockup.