LuaSyncedCtrl: add callouts SetFeatureVelocity and SetFeaturePhysics
these work like their SetUnit* counterparts, reference the wiki docs

LuaUnitDefs: deprecate the "drag" key, was an internal variable

related to the change below: Set*Physics now takes three extra number
arguments (Set*Physics(..., number drag.x, number drag.y, number drag.z))
to control how much an object is affected by drag forces

SolidObject: model physical drag, use it for features and skidding units