Repostion Slider Bar when Slider size changes

Reposition Slider Bar when Slider size changes by adding
Slider:OnBoundsChanged(). Had to add empty body for
UpdateBarFromValue, because it may be called from constructor.
(Should fix that, but Gwen seems to end up calling virtual
functions from constructors also otherwise.)
Merge pull request #82 from epajarre/slider-resize

Repostion Slider Bar when Slider size changes