Restructured place & break event handling a little
Cleaner now; onBlockPlace() and onBlockBreak() implementations no longer need to call the superclass method, and no longer need to check if the event is cancelled (it never will be).
Big solar cell overhaul
- Block light sources don't help anymore; light level is calculated only on sky visibility and time of day.
- Added PV Cell; solar cells now require a PV cell, which wears out over time
- "Basic Solar Cell" is now called "Basic Solar" (item ID unchanged)
- Added Dense Solar, generates power at up to 4.0 SCU/t

Also added "/stb redraw" command; op-level command to force STB block redrawing