Selection object, to control different contexts and selections
SelectionCommand and SelectionEvent relies in new selection object
Added selection object to model
Selection test
Updates after API change
Updates tests after API change
SceneView uses selection contexts
pushContext->pushStack, to avoid confusion with the term "context"
Clipboard uses context. Cut test updated
Fix after rebase
Updated Tracker
Added basic implementation for Google Analytics tracker
API updated
Comments addressed.
Comments addressed.
Merge pull request #668 from anserran/contextmanager

A better managment in selection and context
Merge pull request #669 from anserran/ga

Basic implementation of user tracking system (using Google Analytics)
    Added ConversationSystem, DialogueSystem

    - ConversationSystem processes a TalkComponent, which is added by
      TriggerConversation effects. They are display-independent.

    - DialogueComponent processes DialogComponents, which can specify
      callbacks. Currently they all have the same style; this will
      be addressed in later commits.
Added ConversationDemo

Click on dialogues to choose / dismiss them.
Add a components.dialogue package, systems.dialogues package

Since there are now two types of dialogues (menu and line)