handle interactions w/ explosions

[fluxionary]

this was originally pyrollo#50

1. sign entities shouldn't be destroyed by explosions if the sign isn't destroyed
2. sign entities (and node metadata) should be removed if the sign is destroyed by an explosion.

[wsor4035]

some initial thoughts

[fluxionary]

i'll get back to this at some point, it's on the todo list

[Niklp09]

@fluxionary Can we merge this?