refactor: Use QMainWindow as a top level window instead of QQmlApplicationEngine
refactor: Reimplement the canvas with QWidget

Now we can utilize full pixels under HiDPI environments and get rid of
the hack related to tablet events (LTabletEventDispatcher).
feat: Display actual pixels of an image at zoom level 0
Merge pull request #1 from kazniatory/feature/hidpi-canvas

feat: Utilize full pixels of the canvas under HiDPI environments