Add a few notes about design

Might be useful for a thesis or future documentation.

design_notes/README.md

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+A few files related to the design. Currently notes for myself, which do not
+necessarily make sense without implicit context.
+
+One day, we should have a proper `docs` directory with coherent documentation.

design_notes/layers.md

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+Layering:
+
+File format: BIRD OSPF style
+	TODO: spec
+		- Two sublayers: syntax, semantics (what attributes are defined for what)
+	Ref: BIRD's source code?
+
+Parsing layer: create own objects from the OSPF state dump.
+	FIXME: currently does not exist and everything is ugly
+
+Graph layer: get two maps: ref and current
+	compare using some dicts, probably
+	Output: the graph, differences between maps
+
+Visualisation:
+	Vertex and edge positions and styling
+	Network reduction (TODO: here or before?)
+	TODO: File format: use BIRD style, or something else
+		- We need to be able to write this
+	Underlays
+	Interactions (SPOFs, trees, path finding)
+	Map editing mode

design_notes/map_differences.md

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+Reference map vs OSPF map:
+
+Both same: super
+Router missing in reference map: was not added, pls add
+Router missing in OSPF map: probably down?
+	- The rest should be connected, otherwise this is weird (our bug?)
+Network missing from ref: this is strange, maybe reconfiguration?
+	- If behind a previously unknown router, it's less strange
+Network missing from OSPF: All links down? Seems wrong.
+Connection missing from ref: should not happen unless reconfiguring
+	- We can draw that without config as straight line. This feels much safer than autopositioning networks or routers.
+Conn missing from OSPF: link down, quite likely.
+
+
+
+Analysis functions:
+- SPOFs
+- Routing trees
+	- Note: Link costs are asym? Cost is from router to network, and network assumes all routers are there?

design_notes/toolkit_choice.md

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+Qt vs Gtk:
+- The main two multiplatform toolkits. Not going away soon, many examples, big communities
+- Qt chosen because it has better documentation (at the first glance), I have a tiny knowledge of it and Gtk feels a bit stubborn…
+	- Interlinked, not just separate concepts. I found the wrong drag'n'drop at the first time…
+	- The complete drag'n'drop example already part of documentation (examples/widgets/graphicsview/diagramscene)
+		- IDK whether it will support image export…