Create a Balloon Panel sample

[AnnaTomanek]

Let's create a meaningful SDK sample that would show how useful the Balloon Panel plugin can be without Accessibility Checker.

Features we'd like to highlight:

• Setting custom content in the balloon panel.
• Automatic balloon repositioning on editor scroll (classic editor).
• Alignment to the borders of the viewport in case of classic editor.

As for the content of this sample we're looking for propositions, so feel free to propose something that would make sense to end user/developer.

Related issue on CKEditor tracker: http://dev.ckeditor.com/ticket/14520