Offer `static` version of `isLoaded`

[mokagio]

While working on this 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