Making bridges indestructible for now.

Making bridges indestructible.
Rationale: 
-many maps don't work when bridges are destructible (most or all of the crossings are bridges)
-bridges will be repairable in the future. At this point, an engi can come and re-open it, but in the meantime, they shouldn't be destroyed.
-if someone wants to make a map with destructible bridges, they still can, by editing the .yaml themselves. This should be the default, though.




(FWIW, bridges were not destructible in C&C 1.)
replaced all explosion numbers with names

Explosions all have a name description now instead of a vague number.
Sniper range reduced from 7 -> 5.5

Sniper range of 7 seems ridiculous, since it's a one-shot kill, and you can build multiple. (I support building multiple). Surprised I never noticed this issue before.
The range in C&C 95 was 5.5. 
The ScanRadius is set to 5, which is probably why we haven't noticed any OP problems. 
It's also a bit ridiculous since the unit's sight radius is only 5.
housekeeping: moveing artillery BuildPaletteOrder
fix Chinook "returning to base" problem

Chinook had "Re-arms at: HPAD" entered in its rules, even though it has no weapons. This is what caused it to return to base when you hit 'F' instead of land.
Now it lands again when you hit F. *phew* so happy :)
housekeeping: moved arty over some more in menu
Merge pull request #3035 from psydev/patch-1

Making bridges indestructible for now.