Update data flow diagram

- Incorporate Export Device and describe flow as stated in docs
- Remove OnionShare, since it's not documented (for now) and will
  likely remain secondary, while potentially adding a lot of
  complexity here that would confuse rather than help
- Use the same visual category for the printer as for the Export
  and Transfer Device, so that the printer does not look scary
  red
- Add USB icons for Transfer/Export device; printer icon for
  printer
- Small visual fixes

Resolves #4670