This commit also makes snap to grid and snap to vectors work off-canvas.
Since we now have off-canvas viewing, it just makes sense that snapping
would work there too.

Note that I disable snap to grid when "Show All" is OFF. I am actually
unsure this is right (as "Show All" is a view action, and we usually
don't change behavior based on view actions; for instance snap to guides
are not disabled if guides are hidden). Yet I noticed we do this in
various other features when off-canvas. We kind of use this view flag as
a switch for features working off-canvas (for instance, color picking
works off-canvas only when "Show All" is ON). So let's keep the same
logics for now at least.

Snap to guide or snap to vectors will always work though, because guides
and vectors are always visible off-canvas (even when "Show All" is OFF).
They always have been (visible, not snappable off-canvas; now they are
both).