Changed implementation to use the editors editable UI view

[alexeckermann]

Previous implementation was based on an incorrect assumption in the ClassicTestEditor that does not match the non-test ClassicEditor interface (see: ckeditor/ckeditor5-core#137). The plugin will also test its assumptions about views and extending templates to prevent raising unnecessary errors and breaking end-user usability.

• Note: CSS selectors are now only added to the editable view. This makes the CSS scope simpler.
• No impact on previous implementations, full support of provided CKEditor editor classes.

[alexeckermann]

Theres still some issues with extending the editable view template on BalloonEditor. The balloon editor renders its editor.ui.view.editable in the editors constructor scope meaning that no extensions can be made during the Editor.initPlugins scope which happens thereafter.

It looks as though extendTemplate may not be possible if we cant presume that views aren't rendered before initPlugins -- an incorrect assumption on my behalf. I've raised this with the CK team and will wait for advice.

Having the tests not reflect actual editor integration consistently is a problem and gave false validation of the 0.2.0 release. If this impacts any current production work I recommend targeting tagged version 0.1.0 and wait until 0.2.1.

This may mean that this plugin may have to manually manipulate the DOM elements in its operation. I was preferring to use in-built CKEditor methods, like extendTemplate, to do this as managed views may clear and/or modify attributes like class without notice.

Closing PR because this needs more work.