Offer `static` version of `isLoaded`

While working on [this](https://github.com/Automattic/simplenote-ios/pull/1401) Simplenote iOS automation PR, I found myself implementing a `static` version of `isLoaded`. That's a functionality that we'll need in different screens across our projects, so it would be great to push it down to the `ScreenObject` base class.

One obstacle in the way is that, currently, the `init` method makes the tests fail if the element is not found. I think we should change this to simply throwing an error, or even not checking that the screen exists (maybe) to:

1. Allow a `static` `isLoaded` implementation that internally initializes and instance and calls its `isLoaded` instance property
2. Be less opinionated and let consumers deal with the screen not being available at init time