Add a test that shows the DOM change failure

The icon should switch from blue to red and back, but does not in IE
because the DOMelement has disappeared, as in issue #561. Instead, the
mouseover event is fired on any mouse motion in the marker. In addition,
in FF & Chrome the mouseover and mouseout events are continously fired
on any movement, which gives the correct visual results but causes
a lot of superflous event handling.
Reuse DOM object in Marker.SetIcon, fixes #561

If there already exists a DOM object for this marker, reuse it by setting
a new src and style rather than creating a new one.
Make test/example actually check ondragstart/end
Ensure styles are set even when reusing DOM.

This fixes the error found in review.
Merge pull request #1726 from Norkart/reuseIconDOM

Reuse icon DOM