Slider: Knob position does not change after resize

[mojoaxel]

The slider widget should react on resize events. When the knob is e.g. at position 0.25 and the slider has a width of 100px the knob should be drawn at a position of 25px. When the widget gets resized e.g. to a width of 200px the knob should change its position in this case to 50px. At the moment it doesn't.

[fastner]

Fixed in Pull Request #35