[Nc-GUI] Improved management of events on the GUI.
[Nc-Graphics][Stability] Removed the static instance of GL::State::_current and put a SingletonPerThread for the GL::State.
States are initialized when a new GLContext is created.
This method is much more stable, but could have bad effects on performances.
[PluginFileFormatCollada] Add alpha color on vertices.
[Nc-Graphics] Correct missing initialisation of the scenegraph into subwindows.
[Demos] Correct demos to work with the new events system of the GUI.