Fixed Chests game cave

savegame_variable already has the "b" prefix.
Fix for waterfall cave chests
Dungeon 7 fixes
Gelidrak (dungeon 7 boss) fixes
Dungeon 9 fixes.

The room with the children on the retro level is still broken.
Mystic mirror / waterfall fixes
Dungeon 8 Agahnim fixes (partial)

I needed to comment out the set_position() in on_created() to get this battle
to work. The initial on_restarted() call will start a fade out transition, and
it seems this transition never calls it callback when the sprite is off screen.

There's also still a problem with the fireballs sometimes sticking to the edges
of the map.
Partial Ganon fixes, still unplayable.
Properly fix children's hint in dungeon 9
Fix Gelidrak arrow regeneration
Work-around for christopho/solarus#225 no longer needed
Merge pull request #47 from pdewacht/master

Dungeon 7/8/9 fixes