graph: Redesign nodes and ports

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.

src/ui/graph/mod.rs

@@ -20,6 +20,8 @@ mod node;
 pub use node::*;
 mod port;
 pub use port::*;
+mod port_handle;
+pub use port_handle::*;
 mod link;
 pub use link::*;
 mod zoomentry;