Rework station and line painting

[gfgit]

Adapt to new station schema.
Possibly do not rely on QGraphicsScene because it forces loading on main UI thread and loading in chunks seems difficult.
Since we are not interested in view transformations (except zoom) and parent-child relations of items, it seems an overkill.

New graph types needed:

1. Single station
2. Single line segment
3. Complete line view (multiple segments concatenated)

Encapsulate in a viewer widget (which can be embedded in main window or separate window), statusbar and toolbar and a model worker class.

#2 #1