Backspace handler crosses editable boundaries

[Piotrek Koszuliński]

Steps to reproduce

This scenario may work only in Chrome, because in other browsers it may not be possible to make a selection crossing editable boundaries.

1. Open ​​http://sdk.ckeditor.com/samples/simplebox.html
2. Triple click in the simple box header.
3. Press backspace.

Expected result

Only content of the header was touched.

Actual result

Text from the simplebox content was moved to the header.

Other details (browser, OS, CKEditor version, installed plugins)

This issue is reproducible because of #14353. Once that issue will be fixed, this one will only be reproducible from code, but will still be valid.

Reported in ​http://stackoverflow.com/questions/34992972/ckeditor-titlebody-simplebox-example-widget-select-title-text-and-remove