Add origin keyvalue to func_dustmotes and func_dustcloud

[lenship2]

These entities seem to both require an origin keyvalue set to 0 0 0 to function properly.

[TeamSpen210]

This should be in the BModelParticleSpawner FGD base, since that's common to specifically these two weird entities. It also shouldn't be readonly, I already tried that upstream but it needed to be reverted (TeamSpen210@6cc029a), since then you can't edit it in Hammer to fix existing entities with incorrect origin values. Really, the fix for Strata should be making the entities function when they're not at the world origin.

[lenship2]

In this case, I think the readonly is justified because of Strata's 'Reset' keyvalue button. The button applies to readonly keyvalues, meaning that if incorrect origin values are set a user could just press 'Reset' to fix it.
I could however close this and implement the changes in BModelParticleSpawner FGD base if needed though.