Remove DeclarativeTab class

There are lot of small changes done. DeclarativeWebContainer
uses now only title and url from the active page and those
are also exposed to QML side. DeclarativeTabModel owns now one Tab
instance that matches to the current active tab. This is the tab that
is pushed back to model data when a new tab is activated or an active tab is changed.

Hope this simplifies the structure a bit. There are still parts that I'm not satisfied but it's getting better. 

One ugly part that I don't like is that we have some loading related parts declared in QML side while they should be implemented in C++. As far as I remember only blocking element there is the ConnectionHelper. We could deal with that through meta object or by adding C++ interface for it.