Extension to delay page loading

[André Stösel]

This extensions allows you to delay page loading (after midori startup) until you actually want to use the page.

[André Stösel (ivaldi)]

A better version of the extension.

[Cris Dywan (kalikiana)]

I committed with some fixes:

- Avoid .get() and ,set() in favour of API, otherwise you get no type checks.
- Setting load-on-startup object data is really an implementation detail that happens to work - nobody will notice if the underlying code in main.c changes. I placed a FIXME there and changed it over to change the preference for now.
- Avoid var