Option to fix Rect ratio when resizing

[zhiayang]

eg. holding shift while dragging the resize handle will keep the Rect ratio constant, or even holding ctrl+shift will force it to be square.

ps: i might implement this

[rexim]

@zhiayang sure, feel free to ignore snapping while implementing that feature. Because I think the structure of the responsible for snapping will change a lot in the nearest future.

[zhiayang]

actually i think i'll do it after tonight's stream, cos i'm scared of merge conflicts :tsodinFeels:

[rexim]

Resolved by #1158

[zhiayang]

unfixed by #1170 PepeHands