Allow 'shift'-modifer after dragging an assistant handle to snap lines.

This gives horizontal snapping to most straight-line assistants.
The radial assistants will have the third handle snap to 'perfectly circular'.

Shift is a bit of an unfortunate choice given the dis-merge node function was
at one point also on shift, but shift-snap is similar to the other two tools
in krita(straight line and gradient tool), but that had been disabled anyway.

CCBUG: 332527
CCMAIL:kimageshop@kde.org