Find a nicer way to display non complete traces

[HazAT]

If there is not top-level transaction/span it almost looks like an error, maybe we find better way to visualize it. (Also the platform icon then is missing)

[dcramer]

Two things:

• For the icon, I think we should make it a grid, that is 1x, 1x1, 1x2, 2x2, showing up to 4 platform/services present in the trace.
• For the missing parent span. We could probably make a panel that expands with more details?