[BUGFIX] Handle opacity for IE in prototype.js

The new prototype.js sometimes returns a string as opacity value
instead of a float. This causes problems with the starting
animation for dragging an item, making it completely hidden during
dragging.
Forcing the result to be a float results in a correct opacity.

Resolves: #58053
Releases: 6.2, 6.1, 4.7, 4.5
Change-Id: I7811dec578f5e3222fd8fc95145c1e0cdbedb21f
Reviewed-on: http://review.typo3.org/34329
Reviewed-by: Jigal van Hemert <jigal.van.hemert@typo3.org>
Tested-by: Jigal van Hemert <jigal.van.hemert@typo3.org>