Snapping of widget positions

[janezd]

Continued from biolab/orange3#4154.

A button for recomputation of all widget positions is probably not feasible. What may be feasible is snapping when the user moves widgets. We could emulate snapping in drawing tools, like Illustrator. Canvas could do the following.

• For horizontal snapping, it

  • checks if one of input or output connection lines is almost horizontal (this does not mean that the corresponding widgets are aligned, because there may be multiple connections, so they are not vertically centered with the widget). If so, it snaps the widget position so that line is horizontal.
  • If there are no such lines, it checks whether there is a widget with a similar y coordinate and snaps to it.

• For vertical snapping, it checks whether there is a widget with a similar x coordinates and snaps.

However, we said that while this would be very nice to have, it is not a priority. Can be implemented when @irgolic has nothing else on his list. :)