[sailfish-browser] Move SettingManager to ownership of DeclarativeWebContainer

In case there is some initialization steps that requires that the first
web page is created, we can use DeclarativeWebContainer::setReadyToLoad
for handling those.

Initially added BrowserReady event that was send when we were ready to load. As status of the clear private data was needed before setting manager was initialized, decided to move the setting manager to the ownership of the DeclarativeWebContainer. In future, we can add an event like BrowserReady that would be send once from setReadyToLoad to let other components to do their initialization steps knowing that gecko is ready.