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.
