Use position:fixed instead of position:absolute

This prevents a wrong position of the dialog when there is a different
scroll position like on an URL with fragment, for example
Special:Version?setlang=de#Installed_software

Change-Id: Ic10c6b13461a259d87fd43a718acdd1421193b6c

resources/css/ext.uls.dialog.less

@@ -1,7 +1,7 @@
 @uls-dialog-width: 480px;
 
 .uls-dialog {
-	position: absolute;
+	position: fixed;
 	z-index: 500;
 	display: none;
 	background-color: #fff;
@@ -31,7 +31,7 @@
 	opacity: 0.87;
 	display: none;
 	z-index: 300;
-	position: absolute;
+	position: fixed;
 	top: 0;
 }