Fixing zoom-slider issue with jumping thumb.

To avoid a circle call of the events change and zoomend a property zooming
was introduced, whichs indicates that the zoom operation was caused by a slider change and
that another setting of the slider value based on the map is not
necessary.
Cleanup code, non-functional.
Correct ZoomSlider behaviour and appearance.

This corrects the behaviour and appearance of the ZoomSlider element when
it is a child of a GeoExt MapPanel.
Merge pull request #190 from chrismayer/fixing-slider-issues

Fixing zoom-slider issues.

Thanks for reviewing @marcjansen