Fixed: CPPopover animation issue

Previously, when opening a popover after another one, the CPPopover was blurred.
This occurred due to some CSS Transitions specification :

"Since this specification does not define when a style change event occurs, and thus what changes to computed values are considered simultaneous, authors should be aware that changing any of the transition properties a small amount of time after making a change that might transition can result in behavior that varies between implementations, since the changes might be considered simultaneous in some implementations but not others."

The fix consists to access to a transform value of the style of the _DOMElement to force it to recalculate the values.

More information here : https://code.google.com/p/chromium/issues/detail?id=388082

Fixed #2143
Test app in Test/Manual/CPPopover