Make the linkings between touching occupancies visible and clickable

[kmer2016]

Description and goal

Two occupancies can touch: one train leaves the track just as the next one takes it. Their linking then has no space left to be drawn, so nothing shows on screen, and there is nothing to click to create it.

Design proposed by @maelysLeratRosso and @Signez:
#18000 (comment)

Acceptance criteria

• A linking whose start and end times are equal is drawn across the track
• It keeps the colors, the stripe and the states of a horizontal one
• It can be hovered and clicked on its whole footprint
• A storybook story shows it, both suggested and created