Extend visualisation.ospf to include styles

A concept/idea stage atm, but seems simple enough and nice.

The builtin style might want to have a sigil. The default possibly too.

The core idea is that everything that is not annotated uses the
"default" style, which is user-tweakable and defaults to the "builtin"
style, which makes sure everything gets drawn in some sane way.

Implementation-wise, a first annotator would lay out the vertices and
shortcut them possibly, second one would lay out the
unpositioned-but-shown vertices (in a similar way to the
random_positioner), third would process the styling graph and annotate
with styling directives. The actual visualisation would then just read
the annotations and topology¹ and draw that into the QGraphicsScene.

¹ We might need to devise a different representation to make sure thaat
the shortcuts work reasonably.

For a very broad overview, something akin to:
	style default
		hide
	style important
		show
		pen color red
could be used.

The styling should possibly be in opposite order: object property
params. This way, we can quickly ignore the irrelevant properties (like
line width for vertices)

An extreme but probably workable idea would be to reuse a TopologyV3.
While the families and addresses make zero sense, it could represent the
reduced graph. However, we would need some way of mapping the new
vertices/edges to the old ones (for Dijkstra &c.), so it is probably
just better to create an Annotator-driven "overlay" in the existing
AnnotatedTopology (reuse a subset of VertexIDs, maybe create own Edges.
Those objects can exist without a TopologyV3, so it is fine. Also, at
that point we do not need to use too-reasonable objects in the
Annotations.)

Regarding the "icon router bleg.svg": We will probably want to only
allow files from assets in the beginning, so that the visualisation
files are portable without any restrictions. In future, we could have a
visualisation-relative resolution and/or directives to add other paths.
(maybe with prefixes, Pelican style)

The styles should possibly be top-level directives, not l2, so that they
are reusable between presets (in the same file)

visualisation.ospf

@@ -84,6 +84,23 @@ visualisation default
 
 	network [192.168.15.1-10]
 		position [200 800]
+		hide
 
 	network [192.168.15.7-24]
 		position [200 500]
+		style important
+	
+	edge
+		source router ...
+		target woo woo'
+		style important
+
+	style default
+		inherit builtin
+	style important
+		inherit default
+		pen width 14
+		pen color ff0000
+		icon router bleh.svg
+		icon
+