Fix system info view issues

This fixes several issues in the system info view, some reported (#2741, #2743) and some not:

- Clicking on the background will now show the system information again
- Selected body in system info view will now properly follow the navigation target (in current system) or the hyperspace target (in remote system). It will also notice when a remote system becomes the current system due to jumping.
- No more assertion for system info view of unexplored system.
- Clicking on the selected body of the current system again will clear the navigation target.