Tree generation code sometimes set the non-existent growth state 7
This could also become manifest in snow covered trees below the swnow line in arctic climate and similar effects
This includes 2 fixes
-Fix: [1048596] Monorail and Maglev sounds are swapped (r511)
-Add special case to load the jackhammer sound (r478)
The rest are cleanups und enumeration to make merging possible/easier
