Resolve bug/72-imageSlider-doesnt-update

Closes #72 (closed)

The problem appeared after a new image was pushed the the ImageSlider-props, which lead to a wrong display of the slider (not all images were visible). (That only appeared, when ion-slide and ion-slides components are imported correctly.) Solution: Now the ImageSlider has a :key="images.length" attribute. So the ImageSlider re-renders as soon as images.length changes. That emulates the correct behavior, that the component re-renders on props-change. Besides that i removed some obsolete code parts and changed some minor things.