Fix missing/incorrect propTypes on geppetto-showcase

After the work done on #246.
We realized a couple of new issues related with the showing of propTypes which require further investigation.

For example, the PlotComponent has the following props: https://github.com/MetaCell/geppetto-meta/blob/ad6693652a23cdc429979d43b996ae1455a2c1e5/geppetto.js/geppetto-ui/src/plot/PlotComponent.js#L380

But the prop list looks like this in the showcase: 

Fields such as `PlotComponent.plots.x` and `PlotComponent.plots.lineOptions` are not displayed in the showcase.

Checklist:

- [ ] Add type and description to connectivity viewer > names > name attribute:
- [ ] Add type and description to VR canvas > instances > name attribute
- [ ] Fix incorrect props for connectivity viewer > layout
- [ ] Fix incorrect props for dicom-viewer
- [ ] Fix incorrect props for plotcomponent > plots
- [ ] Fix incorrect props for list viewer > columnConfiguration

Start with geppetto.js/geppetto-ui/src/plot/PlotComponent.js and compare its declared props with the prop list rendered in geppetto-showcase. Investigate the listed connectivity viewer, VR canvas, dicom-viewer, plotcomponent, and list viewer entries, then verify that each checklist item shows the correct type, description, and nested props in the showcase.

Written by the indexing model from the issue text.