Added the feature to load an expression from file.

 On branch master
 Changes to be committed:
	modified:   build-calculator-Desktop-Release/calculator
	modified:   calculator/calculator.cpp
	modified:   calculator/constants_dialog.hpp
	modified:   calculator/expression_edit.cpp
	modified:   calculator/expression_edit.hpp
	modified:   calculator/main_window.cpp
	modified:   calculator/main_window.hpp
	modified:   calculator/normal_view.cpp