While this is work-in-progress from custom-manipulators branch its stable so adding into 2.8 so we don't get too much out of sync. - ManipulatorGroupType's are moved out of the manipulator-map and are now global (like operators, panels etc) and added into spaces as needed. Without this all operators that might ever use a manipulator in the 3D view would be polling the viewport. - Add optional get/set callbacks for non-RNA properties Needed so re-usable manipulators can control values that don't correspond to a single properly or need conversion. - Fix divide by zero bug in arrow manipulator (when moving zero pixels).