Establish testing APIs for editor UI

[Garry Yao]

We could encapsulate APIs for functional testing, consider the following basic ones:

1. Toolbar button clicking;
2. Dialog controlling;
3. Document selection and keystrokes;

[Garry Yao]

The patch implements Toolbar button clicking functionality while there's two noticeable changes :

1. Ideally it's assumed that the testing API wouldn't touch any source codes, but in this special case where currently there's no way of understanding the UI objects' internal status( e.g. when a panel is loaded ) from outside, so I'm proposing of adding necessary event hooks for making it easier to work with them.
2. It's also notice that the way how stuffs appeared on toolbar is designed make it very hard for tracing the related command,ui element, and toolbar items, which has also been identified by #4137, but here we need further changes in 'core/ui.js' to connect them tightly.

[Garry Yao]

Ticket Test added at : ​http://ckeditor.t/tt/4252/4252.html.

[Garry Yao]

Adding dialog controlling( population ) API, corresponding TC added at : ​http://ckeditor.t/tt/4252/2.html.

[Garry Yao]

Adding editor selection API, corresponding TC added at : ​http://ckeditor.t/tt/4252/3.html.

[Garry Yao]

Updating editor selection API, corresponding TC added at : ​http://ckeditor.t/tt/4252/4.html.

[Garry Yao]

Updating editor typing API, corresponding TC added at : ​http://ckeditor.t/tt/4252/4.html.