[FEATURE] Enable closing of ExtJS FlashMessages

Some of the FlashMessages have such a big space requirement that users
have to wait, until they disappear to continue working in the backend.

This patch makes the ExtJS FlashMessages disappear, if the user clicks
on close icon in the message header.

Change-Id: I270dc0b6b4cfef945aa5802abcf586671db63e8b
Resolves: #27747
Releases: 4.6
Reviewed-on: http://review.typo3.org/2956
Reviewed-by: Steffen Gebert
Tested-by: Steffen Gebert
Reviewed-by: Kay Strobach
Tested-by: Kay Strobach
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter