Improve webview unit tests

Check tab order based on url after adding a new tab
Fix real tabs count to work correctly over browser restarts

This commit exposes liveTabCount to QML and that could be bound
to e.g. a settings value.
Merge pull request #70 from rainemak/liveTabCountFix

Fix real tabs count to work correctly over browser restarts
Move tabModel property to C++ side
Rename TabCache to WebPages

Ownership of TabCache will be moved to DeclartiveWebContainer
in following commit(s).
Move WebPages to DeclarativeWebContainer's ownership
Merge pull request #71 from rainemak/webPagesCleanup

Web pages cleanup
Merge pull request #68 from rainemak/improveWebViewUnitTests

Improve webview unit tests
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