Correct OpacitySliders behaviour and appearance.

The OpacitySlider class only works currently, when the slider is rendered stand-alone and not as a child of a mappanel (see the examples, e.g.).

When the slider is added to a mappanel

* it will not respond correctly to mouse events (not letting go, capturing area way to big)
* the configured width/height is not respected
* the rendered dimensions 'jump' on first click on the slider

This pull request fixes all beforementioned issues. Tests pass in FF 24 and Chrome 27. I do not have a IE 8 or later at hand currently.

f8d7d5b and 907d7e2 might be of relevance to #184/#190.

Please review.

(and please ignore the name of the branch. I wasn't thinking when I created it)