Add no shadow option? #192

For making screenshots on Mac it is extremely annoying to have the shadows around windows. It also makes the trim.py not work.

My current solution is to have two branches with small fixes for hiding shadows. This gets annoying when Orange/canvas/applications/canvasmain.py and Orange/widgets/widget.py change and one has to look for places to insert self.setWindowFlags(Qt.NoDropShadowWindowHint) at.

Could there be an option for hiding shadows in Preferences - Style? Just an idea.