convert floating point values of zoom level to int (#840)
convert floating point values of zoom level to int (#840)

In some cases, arithmetic results (in particular from division) on zoom
levels return floats; however, the PyQt API requires int values to
function correctly. Therefore, explicit conversion to int or integer
division has been added for these cases.

A behavior bug causing sudden crashes was fixed. When a rectangular box
is dragged with the right mouse button, but the left mouse button is
touched prior to release of the right mouse button, the  program
crashed. The root cause seems to be that this deselects the dragged
object so it is no longer available (i.e. `self.canvas.selected_shape is
None`). The new behavior does nothing / cancels the operation if this
occurs.

Added a shortcut for switching file format (CTRL+Y) since I often use
this functionality.
tzutalin/labelImg,tzutalin/labelImg