Determine if system is explored in Sector layer instead of StarSystem.

This pulls up determination whether a system is explored from ```StarSystem``` to ```Sector``` as that is a high level property of a system. Systems within 65 sectors of Sol are not changed, as they are always set non-randomly to explored. Systems beyond that will change due to the random determination moved from the random generator of ```StarSystem``` to the one in ```Sector```.

Thus, **bumps savegame** and is to be merged together with #2738