[TASK] Use language labels in Extension Manager javascript

Replace text in Javascript by language labels. Fix jQuery behaviour
for update, upload and other icons. Replace some text links by icons.
After answering No to update confirmation the mask in the background
is removed.

Change-Id: Id934e4c270897d5810ff68e1f076a91d7ba182fb
Fixes: #42033
Releases: 6.0
Reviewed-on: http://review.typo3.org/15759
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel