Slider: Hover only on the handle

Most styles rely on the handle being hovered, not the whole slider.

[ChangeLog][Slider] The hovered property is set only when the handle
is hovered, not anymore on the groove

Change-Id: I749b076c98fba8e344218e46637ec00d24c0250c
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>