Introduce CSS class to enable lower specificity style overrides

Making `#language-settings-dialog` carry the same CSS class
`.language-settings-dialog` to be able to override it in theme styles
more sanely. Follow-up to Id9c8652820641d6.

Bug: T175295
Change-Id: I1aa69f8701ed6d6cb2ed59cabf11332f2a10d21f

resources/css/ext.uls.languagesettings.less

@@ -74,7 +74,7 @@
 	font-size: 12pt;
 }
 
-#language-settings-dialog {
+.language-settings-dialog {
 	background: #f8f9fa;
 	/* Ensure base font size is same in different skins */
 	font-size: medium;

resources/js/ext.uls.languagesettings.js

@@ -44,7 +44,7 @@
 		settingsMenu +
 		settingsPanel +
 		'</div>';
-	windowTemplate = '<div style="display: block;" id="language-settings-dialog" class="grid uls-menu uls-wide">' +
+	windowTemplate = '<div style="display: block;" id="language-settings-dialog" class="language-settings-dialog grid uls-menu uls-wide">' +
 		closeRow +
 		panelsRow +
 		buttonsRow +