Additive selection mode

[bmatthieu3]

We would like to have a mode (whether it is a shortkey or not) to be able to do additive selecting.
Currently when a shape has been selected, if the user selects another shape, the first selected one gets unselected. In this mode, the first selection would remain while the second shape would be also selected.

[bmatthieu3]

When selecting objects (either from a click or a the release of the mouse in selection mode)
If Ctrl (or Command on Mac) is pressed:

• if the objects are not in the selection add them to the selection
• if the objects are in the selection, remove them from the selection