Allow identifying the context of the different ULS panels within one app

Add the ulsPurpose option. Empty string by default.

This is useful for web application that use ULS in several different context,
and need to distinguish what was the purpose of the particular ULS panel.

An example of usage can be found at
https://phabricator.wikimedia.org/T179402

src/jquery.uls.languagefilter.js

@@ -233,7 +233,11 @@
 		resultHandler: function ( query, results, autofillLabel ) {
 			if ( results.length === 0 ) {
 				this.$suggestion.val( '' );
-				this.$element.trigger( 'noresults.uls', query );
+				this.$element.trigger(
+					'noresults.uls',
+					query,
+					this.$element.parents( '.uls-menu' ).data( 'uls-purpose' )
+				);
 				return;
 			}