Mac: Allow control-click to open contextual menus (like right-click).

Observed on Mac, 1.0alpha (8a5469a, 2019-05-29)

In most contexts on a Mac, using a control-click can open contextual menus on objects (for example in the Finder, and throughout most Mac applications), just the same as a right-click.

For reasons of familiarity and ease of use, it would be ideal if this behavior were present in the Mac build of Inkscape as well.