slider: prevent the handle position being set while the user is dragging it
slider: prevent the handle position being set while the user is dragging it

Ensure that the handle position is only updated by motion events while it
is being dragged by the user.

pwithnall/mx,pwithnall/mx,jonnylamb/mx,clutter-project/mx,jonnylamb/mx,pwithnall/mx,clutter-project/mx,jonnylamb/mx,jonnylamb/mx,pwithnall/mx,clutter-project/mx,clutter-project/mx