use `dX` to move handle instead of `pageX`, which makes handles track cursor at point of click instead of center. add check to disallow movement when cursor is outside of handle.
use `dX` to move handle instead of `pageX`, which makes handles track cursor at point of click instead of center. add check to disallow movement when cursor is outside of handle.

ihmeuw/ihme-ui,ihmeuw/beaut,ihmeuw/beaut,ihmeuw/ihme-ui,ihmeuw/beaut,ihmeuw/ihme-ui