Improve relation rendering.

[yrammos]

While a comprehensive implementation of #20 remains the "holy grail," we can meanwhile pursue a more modest solution to the issue of overlapping slurs and unwieldy metarelations.

As a first step, we could use slur-like (Bezier) curves for relations, adhering to the right-hand column of our Figma prototype.

Steps:

• Replace capsules with slurs.
• Use simple heuristics for collision reduction.
• Stack multiple slurs between the same endpoints (see Figma)
• For every relation that is member of at least one metarelation, implement a metarelation toggle in the shape of a circular mini-button, as shown in the Figma.
• ...

This issue addresses #239.

[yrammos]

@arundhatibala let us keep track of this feature here.