1) All slicer's exceptions are now derived from Slic3r::Exception.
2) New exceptions are defined for slicing errors.
3) Exceptions are propagated to the Plater to show.
It remains to modify the slicing back-end to throw the new SlicingError
exceptions instead of std::runtime_error and to show the other exceptions
by a message dialog instead of a notification.
* Show it on the display same as an Application
* Code refactoring : All related functions moved to the SplashScreen class
* Add a possibility o hide/show splash scree in Preferences
