Added the history feature.

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