Make page & title model extendable

This is a very basic way to extend the page and the title model. I use it in conjunction with custom toolbar menu items. With some menu modifiers I hook the models to the menu to display an icon and a short description for every menu node. I decided to make this pull request to raise a discussion about this topic. A better support for page and title model extensions would be desirable. My approach could be easily integrated deeper in the page publish mechanism but I wanted to seperate it the most possible way to be independant from the current development process.