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 DeclarativeWebContainer reference from DeclarativeTabModel
Merge pull request #69 from rainemak/firstFrameRenderedFix

Replace firstFrameRendered with DomContentLoaded