Enabled to switch the view of the main window.
TODO: Set the layout of the scientific view widget.

 On branch master
 Changes to be committed:
	modified:   build-calculator-Desktop-Debug/calculator
	modified:   calculator/calculator.pro
	modified:   calculator/main.cpp
	modified:   calculator/main_window.cpp
	modified:   calculator/main_window.hpp
	new file:   calculator/normal_view.cpp
	new file:   calculator/normal_view.hpp
	new file:   calculator/scientific_view.cpp
	new file:   calculator/scientific_view.hpp