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/style.css

@@ -37,4 +37,14 @@
 
 graphview {
     background-color: @view_bg_color;
-}
+}
+
+port {
+    padding-top: 3px;
+    padding-bottom: 3px;
+}
+
+port-handle {
+    border-radius: 50%;
+    background-color: @media-type-unknown;
+}