add "wait for element" + "wait for condition" components

component should keep waiting until an element defined by a selector appears (with an optional timeout). This will help prevent workflow errors due to short delays... given that a browser & the internet is a very unpredictable environment.

Similarly a more general "wait for condition" component, where the condition can be specified as a JS function that returns a boolean (true / false) and has access to the window/document object & all other native JS utils.