replace barplot with step line plot; add drag-zoom

Headers/imagecanvas.h

Headers/plotwindow.h

Headers/qcustomcanvas.h

Sources/MainWindow.cpp

Sources/imagecanvas.cpp

Sources/plotdata.cpp

Sources/plotwindow.cpp

Sources/qcustomcanvas.cpp

Sources/worker.cpp

customplotzoom.cpp

customplotzoom.h

imagecanvas.ui

imagerQt.pro

plotcanvas.ui

plotwindow.ui