Implement navigation protection

[alexneverpo]

I made a demo based on #8.

The idea is that F1 user creates a Delegate for each form. And a form with delete tells its delegate whether the form is dirty or not.

By default, if one delegate's currentUrl equals newUrl, there won't be a confirmation.

This is still rough. Please give me some comments. @dmnd

[alexneverpo]

I updated the code. It should be more clear now. @dmnd