Add page objects for UI testing (#2501)

* Update page object of change node

* Support multiple node outputs in UI testing

* Add page object of switch node

* Add page objects of trigger and exec nodes

* Remove unnecessary code

* Update page object of trigger node to select time unit

* Add page objects of websocket nodes

* Support boolean as value in selectWithWait()

* Update page object of split node

* Merge page objects of mqtt nodes to make them same as original mqtt node file path

test/editor/pageobjects/util/util_page.js

@@ -70,7 +70,7 @@ function init() {
 
             var ret = repeatUntilSuccess(function(args) {
                 return browser.selectByValue(args[0], args[1]);
-            }, [selector, value]);
+            }, [selector, value.toString()]);
             return ret;
         } catch (e) {
             console.trace();