Track weapon upgrade level and give multiple upgrades when picking up higher level weapons.

[VoxelStorm Bug Submitter]

Submitted by Norgg from IRC (~norgg on norgg.org)

[Norgg]

When the weapon is dropped it should have its level divided by something like 2 or 3.

[Norgg]

This would mean we'd need to re-work the upgrade logic to work off an absolute level rather than modifying values in place, some of the upgrade progressions might be a bit hard to work out this way? I think most of them will just be multiplications/powers though.

[Norgg]

Implemented in 59fc3a432be4e968dfbd0fb006047520934ac897