Rework libraryTreeView event handling

Changes `libraryTreeView::addEventListener('load')` and similar to
`libraryTreeView::onLoad.addListener(listener, once)`, etc. `once` is an
optional boolean that, when true, causes the listener to fire once and
then be removed. This is implicit for 'load'.

'load' maintains its special behavior of running immediately if the
treeview has already been loaded.

Also adds `waitForLoad()` and `waitForSelect()` functions that return
promises on event completion, since most uses of those events were just
resolving deferreds.

test/tests/advancedSearchTest.js

@@ -35,8 +35,7 @@ describe("Advanced Search", function () {
 		var deferred = Zotero.Promise.defer();
 		o.search();
 		var iv = o.itemsView;
-		iv.addEventListener('load', () => deferred.resolve());
-		yield deferred.promise;
+		yield iv.waitForLoad();
 		
 		// Check results
 		assert.equal(iv.rowCount, 1);