Integrated the file manager class. User-defined constant definitions and program options are now saved.

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