[BUGFIX] In IE9, RTE does not work correctly in compat modes IE8/IE7

[BUGFIX] In IE9, RTE does not work correctly in compat modes IE8/IE7

Problem: When using IE9, the RTE does not work correctly when using
compatibility modes IE8 or IE7. In particular, the style sheets are not
parsed, because they are not loaded in the same order as with the
specified compatibility version.
Solution: Check whether compatibility mode is activated.

Change-Id: I817658ef9338fc4cc35e4997102bfd5fd545b49e
Releases: 4.5, 4.6, 4.7, 6.0
Resolves: #38574
Reviewed-on: http://review.typo3.org/12534
Reviewed-by: Stanislas Rolland
Tested-by: Stanislas Rolland