Revamp the node dragging implementation, moving it into the GraphView
widget.
When a drag is initiated, the node widget's current position is stored.
Whenever the drag gesture is updated, the node widget's position is set
by adding the relative drag vector to the position at the start of the
drag.
A drag gesture on the node widget rather than the GraphView widget was
considered, but this seems to lead to a weird flickering effect when the
node is moved while the drag gesture on the node is active.
To avoid interfering with the drag handlers on the ports, check if the
GraphView drag gesture targets a port, in which case the handler does
nothing.
A bug in gtk4-rs makes the library print errors when accessing an event, this change reduces the amount of errors, as they are only printed when the mouse is over a node now.
