[TASK] Assign category 'Backend' to EXT:t3skin

Extensions are sorted by categories.
EXT: t3skin does not have a default category.

Change-Id: I525c0d22a32441d63c5b8561a009ecf43bd58d56
Fixes: #38510
Releases: 6.0
Reviewed-on: http://review.typo3.org/12487
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn

typo3/sysext/t3skin/ext_emconf.php

@@ -13,7 +13,7 @@
 $EM_CONF[$_EXTKEY] = array(
 	'title' => 'TYPO3 skin',
 	'description' => '',
-	'category' => '',
+	'category' => 'be',
 	'shy' => 0,
 	'dependencies' => '',
 	'conflicts' => '',