Increase z-index of drag shadow for files

The drag shadow is a sibling of "#content" so its z-index must be higher
than the z-index of the content children (navigation bar, app content
and app sidebar) to appear in front of them.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>

apps/files/css/files.scss

@@ -612,7 +612,7 @@ table tr.summary td {
 
 table.dragshadow {
 	width:auto;
-	z-index: 100;
+	z-index: 2000;
 }
 table.dragshadow td.filename {
 	padding-left:60px;