Building charge improvements

Improves AttackTesla and renames it to AttackCharge.
Improvement mostly consists of un-hardcoding two delays which should make this suitable for Obelisk-type defenses as well.
This naming should also make it more clear that [currently] only this attack type supports charging.

Furthermore, changes RenderBuildingCharge default for ChargeAudio to null because I dislike those hidden defaults.
Also added ChargeSequence so the sequence name is no longer hardcoded to be "active".

Supercedes #4703.