This reworks the adding of a port to nodes, to avoid assigning multiple nodes
to the same grid cell when a node which was not the last in its column has previously
been removed. Instead, the grid is emptied and repopulated each time.
This also lets us sort the nodes each time by name.
Finally, this hides the seperator if a node has no nodes, as it is unneeded.
Nodes now have a background using the libadwaita .card style class.
Ports now have a circular handle, which is positioned on the edge of the node so that half of the circle sticks out.
Ports are also no longer themed like a button and don't receive a color based on the guessed media type, in a future commit,
the handle will be colored instead.
This ports the application to libadwaita, enabling us to use the libadwaita stylesheet and
widgets to better implement the Gnome Human Interface Guidelines.
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.
