Proposal: Move UI functions to a UserInterface class
In my opinion it's strange to have the UI input functions hang off the Application class. Do you guys think it makes more sense to have them be part of a UserInterface class?

Something like this instead?

``` javascript
var sketch = context.api()
sketch.ui.input("Test", "default");
sketch.ui.select("Test", ["One", "Two"], 1);
sketch.ui.message("Hello mum!");
sketch.ui.alert("Title", "message");
```

`select` is the one that really stands out to me, because "select" has a very understood meaning in Sketch, to select an object. In the `develop` branch this function is `getSelectionFromUser` which if you just read `sketch. getSelectionFromUser` is very confusing, my first thought is this is a function to get the user's current layer selection.
