Add demo page for new installations

[thinkh]

At the moment the user is starting with a blank CMS, which can be confusing for beginners. It would be helpful to install a simple demo page with a few pages and modules to show the functionality.

Must have

• Add to setup: For new installations the user can choose whether the demo page should be installed
• Add a demo layout to Design -> Layout
• Import a navigation module and a WYSIWYG module
• Add a template that uses the demo layout and the modules
• Upload a few images
• Create 1-3 sample pages
• Add content that explain the features of the CMS
• Export content to separate sql file that is (optionally) used in the setup