Cursor movement during port drag-and-drop on the graph is now being tracked
and a link is drawn from the dragged port to the cursor.
If the cursor is hovering a port the source port can link to,
the second end of the link instead attaches to the ports link anchor
so that the link "snaps" to the linkable port.
Instead of different types for each direction to avoid linking ports
of the same direction, we reject the drop early if directions of both
ports are the same.
The direction check is easily extendable to also deny links between
ports of different media types in the future.
The graph widgets management (watching a glib receiver, adding and removing
Nodes, Ports and Links) currently done in the `Application` and `GraphView`
objects has been extracted into a new GraphManager object, which watches the
receiver instead, pushes changes directly to the widgets, and reacts to their signals.
This seperates widget logic and management logic cleanly instead of both
being mixed into the GraphView, and also reduces the code size for the
Application object.
