Add partially explored state to star systems.
Add explored flag to SystemBodies.

The flag is initialized according to the following rules:

- Can be set explictly for custom system bodies (otherwise rules below apply)
- Stars (and gravpoints) are always explored
- SystemBodies of systems that are completely explored are explored
- SystemBodies of systems that are partially explored or unexplored are unexplored
- Space stations inherit the flag from their host body
Show partial system info for unexplored systems (stars only)
Allow partial system exploration, make per-body explorarion accesible by Lua.
Proper refreshing of SystemView and SystemInfoView.
Ensure that SectorView labels are properly updated when system is explored.
Give members of SectorView::SystemLabels an m_ prefix.
Properly update partially exploredness of StarSystem.