Various viewer improvements: single instance, figures mgmt, sharper image export

- Only a single instance of the viewer can be executed at the same time, trying to run a second instance will reveal the previously lauched instance window. (implementation based on QLocalSocket, QLocalServer)
- Closing a figure window now destroys the figure.
- Double-clicking a figure in the list reveals its window.
- Image exports now have a better resolution (screen resolution x2)