- NEW Imp show all central viewer
    The show viewer actions are not needed anymore
- NEW introducing the dual panel viewer
    Very simple viewer that relies on MedicalImageViewer to
    do the heavy work (for instance the synchronization between the viewers
    is already taken into account by what the MedicalImageViewer set in place).
    Shows how to simply re-use the default axial and 3D viewer
    in a dual panel viewer...
- NEW Visibility menu to set the visibility of a component in a viewer
- FIXED simplified Viewer API
  No need for the numberOfViewedComponent method
- FIXED BitmapViewer and InteractiveSliceViewer can only see SingleImageComponent
- FIXED image component management of visibility in 3D viewer
    and some override warnings
    Thanks to Maxime that prepared most of then visibility menu code
- FIXED add viwer count test + fix numbers of actions and extensions
- FIXED broken tests
    Note that the sleepingwhileworking automatic test are
    now disabled until it is possible to specify a dependency
    of the action with one ore more viewer extensions
- FIXED bug: do not record null viewer is the component viewer's list
    When testcomponents is ran, no viewer extensions are loaded.
    Application::getViewer(..) returns nullptr. As MeshComponent set its
    visibility in the 3DViewer, and there is no 3DViewer, nullptr was registered
    in the list. This caused a crash later on, when refresh() was called.
- FIXED arbitrary viewer in fancy
- FIXED embed the arbitrary viewer in the medical image viewer