Suppress useless pointer and add missing this

Closes #94

See merge request !144

Feature/viewer extension

Closes #32, #93, #92, and #91

See merge request !141

+ CepGenerator comment clarification

That cleans up the Viewer abstract class and this little breach in CamiTK
architecture principle

...not a list of viewer's pointer.

NEW nogui example now successful + nogui test + safeguard around all Application::getViewer(Viewer*)

The nogui application is an example of an single independent program that
uses CamiTK to load an image, apply a CamiTK action and save the resulting image to file.
Safeguards around Application::getViewer(Viewer*) avoid possible crashes while
providing error messages.
Note: the nogui success is now automatically tested as well

As viewers are not independent from core, make sure it is possible
to use any component without the viewers

Using Viewer* is deprecated. Using the viewer name untie the link
between viewers and component
The optimization consist in refreshing the interactive viewer only if its widget is visible
When the widget (InteractiveViewerFrame) becomes visible, the viewer is forced to refresh.

(less text, more separators)

Users can specify a viewer extension file. This will be saved in the preference
and load at initialisation time

+ add a specific description for the viewers and little code cleanings

A new test for generating, configuring and compiling a viewer extension
This test guarantees a CEP can add a viewer extension

All the viewers provided by CamiTK CE 4.2 have now a space in their name
Warning: this may break your component visibility.

+ FIXED issues from different analyzer

STACKED was just a specific case of EMBEDDED

config does not call any getWidget() therefore safeguard the deletion process

Now you can use CAMITK_PRINT_POINTER in order to transform a pointer
of any type to a QString (sometimes useful during the debugging process)

- 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