Add no shadow option?

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.

Start by reading Orange/canvas/applications/canvasmain.py and Orange/widgets/widget.py, then trace how Preferences - Style settings are defined and applied. The work is done when a preference can consistently hide window shadows on macOS and trim.py works without maintaining separate branches.

Written by the indexing model from the issue text.