[BUGFIX] RTE only loads styles from external css file on reload in IE8

[BUGFIX] RTE only loads styles from external css file on reload in IE8

Problem: The script fails to detect that a stylesheet is not loaded
Solution: Check for empty rules and imports arrays of skin and
content stylesheets.

Change-Id: Ie9669ca79e7814805dc5dbcd29f5f65939524071
Resolves: #29234
Releases: 4.5, 4.6
Reviewed-on: http://review.typo3.org/4858
Reviewed-by: Stanislas Rolland
Tested-by: Stanislas Rolland