Add and use a base class for mappanel sliders.

This reduces the amount of copied code and reuses the methods from the base class.

All tests pass in IE8, Chrome 30 and Firefox 24, The examples work as expected in these browsers.

Please review.

PS: The Zoom slider example still shows some glitches when the slider outside of the mappanel is being used in a quick manor. AFAICT this is not related to the changes in this PR, as it can e.g. be reproduced in the [online example](http://geoext.github.io/geoext2/examples/zoomslider/zoomslider.html). I will open a separate issue for this behaviour.