Added rp.constants.VERSION
Added manifest.json:rp.required_version

Mods that have a required_version value set will not be loaded if the installed RP is lower than the required version.
Introduce rp.all_mods

rp.available_mods will only keep mods that are available to be loaded, i.e. not ignored, no manifest errors, not conflicted.

rp.all_mods however contains the whole list.
Workaround/Hack/Fix: stonehearth counts as available

Until I come to a good conclusion, stonehearth is moved into the available mods. I mean, mods can assume that it *did* run and that it did so successfully. It's not a RP mod per se though, and therefore the only mod that fulfills `not manifest.json:rp`.