Placeholder sample is outdated and not useful

[Piotrek Koszuliński]

1. In JS sample we add "CreatePlaceholder" button to the toolbar, but it's called "Placeholder" now.
2. We should discuss whether button name change is acceptable. It will disappear from peoples' toolbars, so if we accept this change, it should be mentioned in changelog and release note.
3. Placeholder sample shows only placeholder and no other CKE's features, so it does not work well as a showcase of the feature. I would add at least undo, redo, format and italic buttons.

[Piotr Jasiun]

Changes in ​t/11079.

[Piotr Jasiun]

​git:d6d304b