Add utility functions for building drop-down library lists

A XUL one for the current use in Advanced Search and an HTML one for
future uses. Sets the value to libraryID and adds data attributes for
editable/filesEditable on the HTML one.

test/tests/advancedSearchTest.js

@@ -202,7 +202,7 @@ describe("Advanced Search", function () {
 				for (let i = 0; i < libraryMenu.itemCount; i++) {
 					let menuitem = libraryMenu.getItemAtIndex(i);
 					// Switch to group library
-					if (menuitem.getAttribute('libraryID') == groupLibraryID) {
+					if (menuitem.value == groupLibraryID) {
 						menuitem.click();
 						break;
 					}