Replace firstFrameRendered with domContentLoaded

The "embed:domcontentloaded" is sent for the root window.
This commit also moves loadTab to C++ side as firstFrameRendered
is not blocking anymore.

Requires: https://github.com/tmeshkova/embedlite-components/pull/48
Cleanup webView qml property from TabModel

Once DeclarativeWebContainer has completed it passes itself to
the TabModel.

TODO: Move TabModel ownership to DeclarativeWebContainer and
creation to the C++ side.