Feature request: transformer key

[benoitlahoz]

Thank you for this work!

Would it be possible to add an option to use a transformer key (Ctrl, Cmd, Shift, Alt) in addition to the button number that would allow panning and/or zooming?
It then could allow e.g. drawing dynamic selection boxes on natural mouse drag, and panning the viewbox when dragging with the key pressed.

Thank you!

[Fuzzyma]

Feel free to submit a pr to add this feature