[FEATURE] RTE: Use HTML5-compliant custom attribute for click-enlarge

[FEATURE] RTE: Use HTML5-compliant custom attribute for click-enlarge

Problem: htmlArea RTE click-enlarge feature adds custom attribute
"clickenlarge" to trigger click-enlarge rendering.
Solution: Use "data-htmlarea-clickenlarge" instead.

Change-Id: I9479fd971eb9d42168731a74e869b8d23a77d373
Resolves: #34053
Releases: 4.7
Reviewed-on: http://review.typo3.org/9083
Reviewed-by: Stanislas Rolland
Tested-by: Stanislas Rolland