Bug fix of button titles.

Button 'content' property was changed to 'title', this is in order to support rendering components as apart of iteration easily as
required by commit [b0a0c499dd550451c1d4] to render the footer buttons of the Modal component.

- Updated all Showcase pages by replacing content->title per button tag.
- Updated showcase ButtonsGroup page with 'Buttons with Metadata' section as now it's easily supported because of the described change.