Fixing zoom-slider issues

According to https://github.com/geoext/geoext2/issues/184 this provieds fixes for the problems with the jumping thumb when the config-option "aggressive" is enabled and the broken behaviour and appearance of the ZoomSlider inside a MapPanel.

  - Jumping thumb: A circle call of of the 2 event-handlers (zoomend and update) should not happen anymore.
  - Broken behaviour and appearance: Analog fix to https://github.com/geoext/geoext2/pull/191 of @marcjansen