Added the layout of the constants dialog.
TODO: Connecting the signals of the constants dialog with the corresponding slots.

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