Replace firstFrameRendered with DomContentLoaded

This PR contains two commits. As there would have been merge conflict between these two, decided two drop them into the same PR. 

First commit replaces firstFrameRendered by domContentLoaded message. DeclarativeTabModel unit tests made to be such that they do not depend on actual page instances. One error found from onActiveTabChanged where DeclarativeWebPage pointer that was pointing to the previous tab was used in when comparing tabId and url. Now we first activate the tab then get the page.

Second commit removes TabModel.webView property that was exposed to QML side.

Requires: https://github.com/tmeshkova/embedlite-components/pull/48