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.
Format params for links and ports are now being watched for in the pipewire connection code.
The parsed media type is then set on the port widget / link object
and they are colored accordingly.
For ports, which were already colored before, this new method of determining the media type
should be more reliable and accurate as this uses the real Format/EnumFormat
params instead of parsing optional properties.
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.
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.
