feat: Select packets (#34)

Implements packet selection on click.

Clicking on a packet now selects it and displays information in the
right sidebar.

Known Issues
- Packets freeze when moving devices: When the simulation is paused and
devices are moved, packets in transit freeze and do not follow the
movement of the edge.

- "Floating" packets when loading a new network with packets in transit:
If the simulation is paused with packets in transit and a new network is
loaded, the packets in transit are not cleared and remain floating on
the screen when the simulation resumes.

Closes #25

Author: pgallino